The film is based on Romeo and Juliet, so Nuka is Tybalt, and Vitani is just there to be a bit of a Plush seller. But I agree with LionKing2001.

In my opinion Nuka and Vitani are like the evil versions of Kiara and Kovu (of course they are not mates!) but they are the opposites such as Scar to Mufasa and Zira to Simba. Just more of a rival- more of a story line. They needed someone of importance to see reasoning at the end. When Kiara stands up to her father saying enough is enough, they needed to have someone close to a main character to agree but from the opposite side. Of course it had to be Kiara's opposite and with Nuka's death the only others were unknown lionesses which wouldn't make much sense in the story line. Vitani was created so that when Kiara has her inspirational speech about how they are all the same, they had a character to step in and agree.
Hope that helps

Lionking2001, I think Celtic kate was right about this question

They wented some one to be the smart one,this is just my opinon.And they also wanted
a girl not a boy.They wanted someone to be funny and pretty happy.She was upset
when Nuka dies.But she turns good at the end.

The point of Vitani is,so that Kovu would have a lion cub playmate,and someone who would keep a really close eye on him,as Nuka wasn't to keen on doing so,as well as a sibling,or sister,in other words Nuka was a very jealous brother.

-Celtic Kate