Nvidia's vdpau implementation is pretty good, but other factors make it
much less attractive for use as default VO. For example, Mesa often has
low quality drivers (mess up things with the presentation queue and the
vdpau API time source). Intel ruins things completely, and we're likely
to run on emulation via OpenGL. Compositing has unknown effects (to me
anyway), but appears to reduce the vdpau advantages.
One important reason to prefer vo_vdpau was that it could do proper
framedropping. Framedropping got fixed for the other VOs, so this reason
is going away.