"If you will walk in my ordinances and keep my commandments and do them... the land will yield its produce and the trees of the field will yield their fruit."
Both Yehuda Halevy, in his book, The Kuzari, and the Rambam have already asked why the reward of those who keep the mitzvot is goodness and abundance in this world, and no mention is made of Olam Haba.
The Kuzari writes ("Part One"), "Are not the anticipations of the other religions richer and more sensuous than 
The promise of reward in the world after death, which no one sees, is a promise lacking real significance.
yours?" And the friend replies: "They are all designated for after death while in life nothing indicates them."
This means that the promise of reward in the world after death, which no one sees, is a promise lacking real significance, compared to the Torah, which promises us reward in this world, in the form of abundant rain and a blessed harvest, in the peace that the land will enjoy and the victory over our enemies. But if we do not walk in the path of HaShem, then the curses will be fulfilled. "In this shall you know that your affairs are not managed by the laws of nature, but rather by Divine will.... And all this shall be fulfilled, all that was anticipated by our Torah came to pass, so there is no cause to suspect that they shall cease to occur." (Kuzari)
The Torah designates reward and punishment for us, which have come to pass throughout the history of the life of the Jewish people. A close examination clearly reveals the payment of the reward promised us in the Torah while we are living a life of kedusha and tahara, contrasted by the intensity of the desolation of the land and our lowly state when we were exiled from our land - all because we sinned. This only serves as a clear proof of the truth of the Torah and its mitzvot, and that he who observes them receives his due.
Furthermore, when we see the blessing and the abundance in our world when the nation is living in completeness, and. Heaven forbid, when we sinned, then we truly encounter, here and now, HaKadosh Baruch Hu, and not in a future which is hidden from us. After all, it is clear that HaShem is watching over us and sees our deeds and acts accordingly, and in this manner we have a constant connection with HaKadosh Baruch Hu. Albeit all this is done in a covert manner and anyone who wishes to deny it will say that everything is accidental, as it is actually written in our parsha, "and if you will walk contrary to me," but this is stubbornness and a small-minded external analysis of the global reality.
Thus, the Ramban describes, on Shmot 6:2, "Reward and punishment in this world, as mentioned in the entire scope of the Torah, are all miracles, but they are hidden. They appear to the onlooker as being part of the natural order of things, but in truth they come upon man as punishment and reward, for were it not for the miracles, man would receive no good in this world as a reward for a mitzvah or bad as punishment for a transgression."
Miracle comes from the same linguistic form as "raise up the banner," which is a flag that flutters and whereby it is possible to identify the location of the king. Thus, the miracles that HaKadosh Baruch Hu performs in our world are for us to be able to identify the presence of his sovereignty. The reward for a mitzvah in this world is a miracle, albeit not via the alteration of the order of creation, but rather a concealed miracle (according to the Ramban), whereby we are committed to HaKadosh Baruch Hu during our lifetime.
Due to a lack of close examination and a lack of understanding of this truth, the nations of the world accused the Torah of materialism, as it does not mention spiritual reward and punishment. This claim is not correct and has already been rejected by the Kuzari: "As to the anticipations of the other religions, our rabbis preceded those religions in this. They are the ones who described Gan Eden and Gehinnom, and listed their measurements by length and breadth, and recounted the pleasures of the one and the tortures of the other, in greater detail than did the men of the other religions that came after them." The other religions also "stole" their spiritual anticipations from chazal, who detailed the spiritual reward even before those religions arose.
This detailing was in the Second Temple period, when prophecy had already ceased and chazal were the ones who were obligated to clarify the spiritual intentions. This being the case, if we delve into the Written Law and the Oral Law, we will find reward and punishment both in this world and in Olam Haba. Here the important novelty is the reward in this world. In this manner, there is an important value to living a complete life in this world, and man's purpose is "to perfect the universe through the Almighty's sovereignty, as opposed to the Christian concept, which caused the construction of a society that was reviled of this world and sought refuge in the next world, such that our world was abandoned to the rule of tyrants who cared only about themselves.
Thus, we find the promise at the end of the blessings, "I led you forth [komemiyut] with your heads held high" (according to Rashi). Chazal clarify the word komemiyut to mean "two statures, like Adam." Rabbi Kook, however, explains that "the body and the soul together are one whole stature, material and spiritual. It was therefore determined that when we keep his laws, HaKadosh Baruch Hu leads us komemiyut, with both our 'statures' complete." It is for this we pray in Shacharit, "lead us komemiyut to our land," - that when HaKadosh Baruch Hu brings us into Eretz Yisrael we shall merit to live full lives here, with both our components, being the body and the soul, in completeness and purity.
All this contradicts the human conceptions that differentiate between these two forces. As noted above, in the Second Temple period chazal expanded on the reward that Man will receive in Olam Haba. Rabbi Kook emphasizes the difference between the reward mentioned in our parsha and the reward about which chazal teach us. The reward mentioned here is a public reward that the nation enjoys together, while the other reward is a personal and private reward. Until the Second Temple period, the nation lived a national life and the root of the national life was "the Divine idea."
In that complete state, the individual was not troubled by the reward he would receive for keeping mitzvot because he was living a complete ideal life with HaKadosh Baruch Hu in this world, just as the Torah promised: "I will walk among you and will be your G-d." It was only in the Second Temple period, when the whole nation did not return to its land, only those who "separated themselves from the impurity of the nations of the land." By then, the light of the Divine idea no longer shone upon life, and that is when Man began asking what he would obtain from all his efforts, so chazal came and began to detail the reward that the individual would receive in Olam Haba.
In our era, when we have merited to return to our land and begin to live life as a people, we truly see that HaShem is watching over us; and again taste the flavor of the more encompassing reward that appears in our parsha, and which was revealed in the period before the Second Temple.
When we merit seeing HaShem walking among us, it is natural that the service of HaShem is not an external burden, but rather bursts forth from the purity of our souls.
This is how we can understand the beginning of our parsha, "If you will walk in my ordinances," which seem to contain an internal contradiction. An ordinance is an external decree that limits man, while walking is a simple flowing movement.
We have learned from Midrash Rabba on our parsha, "If you will walk in my ordinances" - this is the same as, "I considered my ways and returned to your testimonies." (Tehilim 119) King David said, "Master of the Universe, each and every day I would ponder and say, I am going to such-and-such a place, and my feet would bring me to synagogues and batei midrash." King David testified regarding himself that even though he planned to go about his business, his feet would always lead him to the beit midrash (study hall). The feet are the expression of natural walking that happens by itself without planning. If this is the case, then David reached a level at which he was naturally connected to the Torah.
In presenting King David's description, the midrash solves the contradiction mentioned above. The law is external to us and is forced upon us only when we do not connect with our internal independence. However, when we sanctify ourselves, we discover that the law is the truer expression of ourselves and we go to fulfill it naturally. This is what the Torah meant by, "If you will walk in my ordinances."
When one lives on such a level, one merits to see HaKadosh Baruch Hu in our world, and it is thereby clear to us that the material blessings that the Torah promises us are a connection to HaKadosh Baruch Hu, so the individual private question regarding "my reward" does not trouble us.
After the Torah finishes recounting the blessings and the curses, there are halachot concerning different kinds of vows. These are vows that stem from the subjective murmurings of our heart and are not binding. 
In this abundance we discover that HaShem is walking among us.
The question begs as to the placement of these halachot after the blessings and the curses. After all, the blessings and the curses cause us to feel our obligation and privilege to draw close to HaKadosh Baruch Hu and to his Torah. This is not a debate of the details, but rather our general attitude to HaShem as a nation. Suddenly, there are private halachot that are not obligatory except if a person desires on his own, and if that is the case, then there are two opposing mental states here. This line of reasoning leads to the understanding that a person who makes a vow of his own desire could thereby be expressing arrogance and aggressiveness - if he feels he has accumulated wealth and he is willing to forgo some of it in order to "support" the service of the Temple. Certainly, this concept is incorrect and any vow he vows will not be desirable before HaShem.
Even so, when a person knows and understands that HaShem showers material abundance on this world as a reward for our service of Him, and in this abundance we discover that HaShem is walking among us and that He is leading us komemiyut - on both levels - then the voluntary gift that a man offers of his own free will is an expression of thanks for the material-spiritual goodness that he has received. This is complete and supreme service of HaShem. That is why it is specifically this parsha that ends the book of Vayikra, which deals with the sacrifices offered in the Beit HaMikdash.