More fan theory, so don't hate. I respect your opinions, please respect mine.
Anywho, my theory of Vitani's parentage is an interesting one. Tama and Scar. How this happened is quite simple really, Tama unwillingly mated with Scar. Three months later, Vitani was born.
But this doesn't answer the question: Why does Zira have Vitani?
Simple. Tama dies shortly after giving birth to Vitani. She contracts an infection in her womb called pyometra. It can happen in lions (http://www.ncbi.nlm.nih.gov/pubmed/19368254), but it is rare. It's a medical emergency, but since Tama is not a captive lion, the infection kills her. Zira hear's Vitani's hungry cries and comes to her aid, bit more mortherly than normal considering that she is pregnant with Kovu at the time so her hormones are everywhere.
For those who don't know, pyometra is a serious infection of the womb. Without treatment it's a killer. Humans can get this infection as well as cats, dogs, rabbits, rats, ferrets and guinea pigs.
Base by xxmandiixx on deviantart.