Well, it is not exactly specified. So it is open to interpretation. While it looks like she us Ziras and Scars, it is never exactly said. I believe Zira is her mother, since Vitani does refer to her as mother. But I do not believe that Scar is father. It is never said, bot some assume, due to Nukas look. But it is up to you to make your mind.

I highly doubt Scar and another lion ruled together.

It is confirmed that Vitani is the blood daughter of Zira however the father of Vitani was never confirmed. Judging by Vitani's age and the inner workings of a pride, it is highly likely that Scar is Vitani's father. The reason for that is that often lionesses that take the role of reproducing often time it so that all pregnant lionesses give birth around the same time. Since Kovu is probably the same age as Vitani when comparing the size of their youth and since Kovu was the last born during Scar's reign, Vitani was born during Scar's reign. Since no other male lions except Scar would be present at the time since a pride usually consist of a single male lion (unless it is a rare coalition between Scar and a possible outsider; this is however highly unlikely), it is highly likely that Scar impregnated Zira and resulted in Vitani's birth.

Despite all the evidence presented by fellow members and me, there are many other possibilities as well. Perhaps Scar's Reign was rather short, only lasting around a year or two; this might be that some lionesses were already carrying young before Scar took over. Perhaps Scar wasn't the only one running the pride during that time. The possibilities are still out there despite the high probability that Vitani is Scar's daughter.

I dunno, maybe? She doesn't look like Nala or Scar, or any lion king character.

She is Zira and Scar's daughter,but the NalaXScar is just a fan theory

I would say that Vitani is Zira and Scar's daughter. Because, she has Zira's fur colour and Scar facial expression (when she's an adult).