I'd like to add that I tried using an older version of xvba-video (0.6.11), the one that's compatible with the libva in the Arch repos.
I did get vainfo to work; but I had an error on the h264 hwdecode-demo. And when I play any video with -vo vaapi (or vaapi:gl) -va vaapi the video shows a black screen and my cpu goes crazy.
I'm now using the current xvba-video (from the AUR) and thus I need to compile libva.
So this isn't a really helpful post, but think of it as me confirming your problems. :D