I always suggest modeling how it would actually be constructed to get the best results. If you have a concrete floor slab ant the walls are attached to it, then model it like that. Groups are your friends and so are components. Also make sure all of your faces are oriented correctly so that you don't run into problems with back faces facing outward. Really, there's no wrong way to model in Sketchup for V-Ray, there are just more and less efficient ways to work.
If you share your model here I would be happy to take a look and give you some more tips!