You all say her eyes confuse you, well genetics does add in many unique features that are not present in the parents and the eyes are a common one. She seems to be the daughter of Zira, but there is no way in hell she is Nala's daughter, think about the time difference, incubation for a cub is about 3 months and she was born about the same time as Kovu being born at the end of Scar's reign. Nala would have never been able to make that jouney to the Jungle being that she would still have to nurse Vitani during that time and there is also no mention or anything that suggests that she was the daughter of Vitani.
I do think you have a few interesting points, but there was no indication in the film that Vitani had a different mother. She looks like Zira for the most part, the eye color confuses me though. Mostly people are wondering who her father is, like if it's Scar or some other lion. I don't think hunting as much to do with genes, mostly how you're taught. We never see Vitani hunt, really, nor do we see Zira. They live in a harsh outlands where you have to be a good hunter (or find any kind of pagkain at all) to survive. Vitani lived a struggling lifestyle, so hunting is one of things that keep her and her Pride alive.
I think Zira is Tani's adopted mother because Zira has a habit of doing things like that. I think that Vitani's mother is unknown, maybe killed and Zira adopted her. Like when Zira took Kovu in and raised him to be evil. Plus, I don't think Nala's feline ovaries can process something that cute and evil, so I don't think Nala is her mother, plus, Vitani's eyes are too dark a blue. Nala's are light blue.
I'm pretty sure it's NOT Nala. Besides, if Nala was Vitani's mother, why would she fight her? Vitani was raised to hate the pride landers. She has an outlander nose. And, she does fight a lot, maybe to practice for the war. But... What if her mother is not Zira? She turned away from her anyway, and the first one to do so. But Vitani was close to Zira, I think. And besides, Vitani calls Zira "Mother."
In the actual TLK lore, Zira is Vitani's mom. The main tanong is who her dad is, as we never hear about him. We only know that he is a rouge lion who had a lot of girls chasing him. Also, enough of that "Kovu and Vitani are littermates" thing. They aren't even related!