I don't think it's possible to reduce dedicated VRAM (as opposed to Shared VRAM, which takes up some of your system's main RAM). Just out of curiosity, why would you want to? It shouldn't interfere with or "eat up" your regular system RAM.
I dont think it is possible either. What is dedicated is dedicated. Why would be a big question as I have never heard of anyone needing to reduce video memory other than the possibility of ensuring older program compatability