I read all the topic and I think I'm in the good one
To sum up I have core 2 duo 1.8ghz at 2.5ghz , 4coredual vsta motherboard, 8400GS PCI express, 2Go RAM, latest gentoo (from scratch) and i use latest svn mythtv with nova-t (dvb-t) and latest nvidia drivers.
So like you I have display preemted in my mythfrontend when i use vdpau. It crash randomly after 10 sec to 10 min. I use vdpau high quality with bob2x filter. It's amazing my cpu is 96/98% idle. when vdpau is off my cpu is 55 % idle.
I have no problem with mplayer and vdpau (vo and vc). I have no problem when playing 720p mkv or 1080p m4v even with internal mythtv player with vdpau profil. So problem seems come from livetv + vdpau.
LiveTV sd or HD have no pb with others renderers (xv or opengl) but just takes about 50% CPU instead of 2/3 %
I hope it will help