I think that if Kiara had not come back and it had just been Kovu, then Vitani might well have attacked and killed her own "brother". She already attacked Kovu when he tried to help Simba during the ambush.
I think that Nuka's death, coupled with the fact that Zira had always told her that the other side was evil and Kiara was now being kind to her, showing a counterexample to Zira's years of brainwashing, that caused her to finally decide to turn on Zira?
Also, Vitani didn't even seem to show much hesitancy other than "Kovu what are you doing?" when learning of Kovu's loyalties. It didn't appear that she thought over what to do before telling Zira, but went right away, knowing full well how vindictive Zira was and that she might snap and kill Kovu.
Granted, I don't think Vitani would have eagerly killed Kovu, she seemed to hesitate when he was blocking her way to Kiara and Simba, but I think she might have caved had Kiara not talked her out of fighting, and killed her own brother, and that probably would have caused the bloody battle that Zira always wanted, especially as, if Kiara hadn't been there, she wouldn't have been able to talk Simba out of fighting either.