I recommend two things. First (if you haven't already) try posting in the Gentoo Forums. There are a lot of knowledgeable Gentoo people over there

. Second, the proper driver to use is the one that comes with the nvidia drivers (not vesa or nv). I think you should post the output of your emerge nvidia-kernel. That will probably help in determining a solution to your problem. You also might want to try this:
Gentoo Wiki: HOWTO Nvidia Driver Hope this helps!