The setting for vdpau playback is also within mythtv(after enabling it in the webadmin), but it was not set in my install, by default, I enabled it and then got similar issues you experienced.
Yes it seems that streaming from the backend is the way to go (on a fresh beta2 build, it is now enabled), I indicated the change in the mount option, as if you disable streaming from the backend, you need to change the mount options for the frontend to play the saved file.