It benefits from vram, the issue is "how much".
256 should provide no visible benefit in speed that a user will see. In the macbook with the crap 950 built in, ram is precious and you will see huge benifits by putting more ram in so it can share more efficiently. Once it has enough though, you are fine.
Again, the entire screen, as a 2d image, is what.. 7-8mb of memory? Even when moving it around you basically double/triple the amount of ram used so that the edits are treated as multiple screen images which are then moved around. That is a total of MAYBE 30 megs of ram, depending on whats going on and adding in a huge fudge factor. Add in the base OS and the screen/icons/textures and you are talking another 10-15mb. So we are up to a total of 45m ram. Add in a ton of open windows, lets assume 5mb for each (being generous here) and you could open up 11 more before getting to 100mb used. Thats if the windows cache to vram, which I truely dont know.
If you are viewing images that are significantly bigger than the screen (raw 10mpixel images or something) and scrolling around with tons of layers, you potentially could see some slight improvement as those images could be 20mb each, but with only a few open you probably won't.
I'm not trying to be a bastard here guys, but if you are going to quote something as being true you need to be able to back it up. 2d work just isn't system intensive, at least not in the vram department. If you move to 3d where you have hundreds of textures that need to be calculated and mapped, that gets incredibly complex as they all inter-relate and it is all changing 30+times every second. If you are talking about a plain 2d image getting moved around or manipulated but otherwise static, it is, as far as the video processor is concerned, a peice of cake. The real calculation time comes from the cpu chugging on what to change each bit by based on the filter being applied. Storing the before and after in vram is trivial and small.