I haven't used many DS/3DS screen protectors beside Horii's, which are considered the defacto products for both systems, iirc. And while it provides good protection and almost crystal clarity, I feel it compromises responsiveness, but I'm not sure whether it has to do with thickness or resistive touch screen tech. On the other hand, I tried many iPhone screen protectors ranging from cheap, thick, no-name products to high quality micro films, and I never had any problems with responsiveness with either. So I guess this bodes well for the Vita.
Although I have to say, after using my 3GS without protectors for months, I don't think anyone should worry about scratches arising from the mere usage of capacitive screens. That is unless you have dry, shriveled fingers, like old Prole.
I can't say the same for the DS. (I have permenant scratches from the spinning parts of EBA as well *grumble*)