As explained inside, Nitzavim and Vayeilech are combined or separated depending upon whether there is a Shabbat between Yom Kippur and Sukkot.
One of the results of this juggling is that Nitzavim - either alone or with Vayeilech - will be a buffer between Ki Tavo with its devastating and depressing Tochacha and Rosh HaShana. Ki Tavo is close to RH, but we wouldn't want to take the mood it creates into Rosh HaShana with us.
Much better to have Nitzavim always as the lead in to Rosh HaShana. However, if you look at the beginning of Nitzavim, you find a strong echo of the Tochacha that we might have thought we were leaving behind us. Abominations, detestable idols, heart turns away from G-d, gall and worm- wood, G-d's anger will smoke, G-d will erase his name, plagues of the Land, illnesses, sulphur and salt, conflagration, like the upheaval of S'dom, His anger and wrath, forsook the covenant of Hashem, to bring upon it the entire curse, great fury.
How does all of that make us feel any better going into Rosh HaShana? What does Nitzavim give us that Ki Tavo didn't? In short --- HOPE! We've tried to spell it out in greater detail in the Sedra Summary, but here's the gist of it First and foremost, we are presented with the mitzva of T'SHUVA. As bad as things get, we can and will return to G-d. This is the greatest light at the end of the tunnel that can be. And T'shuva will not have to be done all by ourselves.
HKB"H has a vested interest, so to speak, it helping us return to Him. And in helping us return from exile back to Eretz Yisrael. Yes, it is much better if we can do both forms of Return by ourselves. But we will always be given a helping Hand... or more. In the next couple of weeks, we will say over and over that HaShem does not want to punish us, but rather He wants our Return to Him. Very encouraging. And then there is the beautiful con- cept of Free Will and G-d's strong recommendation to us to Choose Life. He is truly on our side. And that's how we want to enter Yom HaDin.