Well, I guessed I figured it out....
In my first try I didn't use COMPAT32:-yes in the SlackBuild of nvidia-driver and I've also replaced the compiled libvdpau with the package from alienbob.
This time I both compiled nvidia-driver with COMPAT32:-yes and kept the SBo libvdpau, rebooted and everything worked as I wanted: correct resolution on tty and full 3D acceleration under X!
I don't know which one of the two changes did the job (I guess COMPAT32) but since now it's not broken I won't try to fix it...
Sorry for wasting your time