Kumiko Ishihara is an example of being both a vampire and a witch. In the Buffyverse, being a witch has more to do with practice and skill than innate power, and anyone of any species can perform a spell. For example, demons Aluwyn and Cyvus Vail were respectively sorcerer and warlock.

Becoming a vampire also does not make someone lose their supernatural powers. Drusilla, for example, was a seer and continued to be one after being sired, and sired Slayers kept an enhanced strength even as for a vampire.

I didn't read the comics after the series ended, thanks for the reply, I already suspected it didn't work innately, as Willow was not born a witch and started practicing becoming very powerful, I also have a fascination for hybrid stories in some shows they are very powerful and being immortal and having magical powers like witchcraft is kind of unusual.

I believe that if a vampire studied witchcraft, like Willow, they could do spells. They would be so cool!