The answer may be "both". It is common practice to separate the site from the building model in two linked Revit projects. However, even if you do not do that, modify the Building Pad type to be a thin vapor barrier layer 2 or 3 mil thick, or even add new layers to represent the crushed gravel, sand, and in some climates rigid insulation, which usually are in place to prepare for a slab.
If you don't need to model these componenets, and will create 2D details, just stick with the vapor barrier approach. Then, add the Foundation Slab element on top of that.
