Yes, vitamin C is perfectly safe to give a cat. You can't overdose. But why are you thinking about giving your cat vitamin supplements? I've heard that vitamin C can be good for acat with URI-problems.
I use Ester-C because it is buffered; regular vitamin C can upset the tummy.
Cats manufacturer their own vitamin C, but many vets feel that they don't necessarily make *enough* for living in these modern times with the stresses of air pollution, noise, etc. I recommend supplementing antioxidants, Vitamin C is a good one. For more info, please see http://www.littlebigcat.com/index.php?a ... atsanddogs