Prime focus as I read it is you are using the telesope as your camera lense, others may have a better explaination. You get the image from your scope to focus on to the CCD chip in the camera directly I believe. I have a fair idea what vignetting is but am at a bit of a loss to explain it and not look like an idiot, if you know what I mean. As I said before, I am very much a beginner here and would prefer the more experienced among the members to give you a better description. With SLR cameras when you look through the camera what you see is what you get, roughly speaking. I know the price difference is a problem, but speaking from my "mistake", it is better to wait just a little longer and go for the Digital SLR. When doing DSO imaging you need longer exposure times and you just cannot get them with the general digital cameras, at least to the best of my knowledge. You have to have a camera with a "bulb" setting which enables you to do long exposures of the dim fuzzies.
I hope I am making sense here, but rest assured, the more experienced members here will give you a much more detailed and correct version of what I am trying to convey.

Regards.

Peter