I wouldn't have thought so, considering she appears to be the same age as Kovu. It seems likely that they are twins, and we know Scar is definitely not Kovu's father.

Even though it's not confirmed that Vitani and Kovu are from the same litter (perhaps Zira became pregnant with Kovu almost immidiately after Vitani's birth!) I get the impression that none of Zira's cubs are meant to be Scar's, considering none of them ever call him "father".

The other thing I find weird is, why would Zira have a cub or two with Scar, and then have Kovu with another lion, and Scar chooses Kovu (the one he is not related to) to be the next king? And apparently forgive Zira for having an affair? Somehow that doesn't make sense to me!

I wouldn't say it's impossible for Nuka or Vitani to be Scar's cubs, but due to certain details I expect it's very unlikely.

I don't think any of Zira's cubs are Scar's. Kovu is NOT though. Vitani is more likely to be some rogue lion's daughter.

I think Nuka is Scars since he looks so much like him

Definitely not!!!