Here's my opinion:
1 - We know Zira worships Scar and would die for him but I didn't see any scene implying Scar had the same feelings for her.
2 - I've never seen a single scene showing Zira and Scar as mates and having a daughter.
3 - Zira never said Scar is Vitani's daughter. Just because we don't know who the poor guy was, that doesn't mean it's Scar.
4 - Zira never said she was Scar's mate or she loved him romantically and they had a romantic/sexual relationship.
5 - In a deleted scene Vitani refers to Scar as "Scar", not "daddy", not "dad", not "father".
6 - Just because Vitani looks a bit like Scar (I don't see it but some people say she does), that doesn't mean she's his daughter.
7 - Zira was more likely to be obsessed with Scar. You can be obsessed with Michael Jackson but that doesn't mean your son is his son.
Is Vitani Scar's daughter? We'll never know but we don't have any kind of evidence to suggest she is.