If it is anything like there other laptop, at the top of the keyboard there is a panel that can be removed - look for a small depression on the right end, and pop it off. Under there you'll find some screws that hold the keyboard in place. Once removed GENTLY lift out keyboard so as to not damage ribbon cable.
I know on older Dells that the screws on the bottom labeled 'K' are holding the keyboard on. I don't know about the Vostro, though.
Hmm... google knows, though-
http://www.youtube.com/watch?v=oSDn64rDzAs