We purchased a new vanity top to replace our 12 year old builder grade piece, however, it does not appear to sit tight on the base. It doesn't really sit "in" the cabinet, and only the back sides seem to make contact. It's a standard vanity top size (49 x 22"), and my cabinets are standard also. What can I do about this? It appears to be fine from the top, but when she open the doors or look under the vanity top from inside the cabinet, you can see the issue. Any help would be greatly appreciated.
In my experience the normal, common way to attach such a top also fills any gaps, since it's good old silicone caulking - White or brown would seem like a good choice here. You can also attach by other means if you like and then caulk to seal and conceal. And you certainly should shim (or sand/grind the high spots on the base) to get it level. If the gap is larger than it seems in the pictures, use backer rod and caulking.
Shims are your friend :-) . Since what you really want is for the countertop to be perfectly level, get some shims (and a good bubble level) and adjust the shim positions until the top is level. Then glue or clamp or whatever you're planning.
If you then have a cosmetic problem, i.e. the shims or some gaps visible, you'll need to get some luaun or other veneer material and cut (and paint) to match.