I believe Scar was already dead when Kovu and Vitani were conceived as they appear to be the same age as Kiara. Therefore, Scar had nothing to do with either of the cubs; certainly he didn't sire them. And people keep mentioning the fact that Kovu was Scar's heir- to this I say, is it not possible that Zira merely saw the resemblance to Scar in Kovu and lied to the others, saying that he was hand-picked by Scar, so that the others would let her cub be the king? Her cub that looked remarkably like the object of her obsession?
I highly doubt Scar would have had any children, and if he did, they would be older than Vitani. So no. I don't think she is his daughter.