It's probably not VDPAU, but CSS.
Sometimes VLC will try to play a disc, even if it is protected by the
Content Scrambling System standard. If the disc *is* CSS encrypted, you will
need libdvdcss2 from vlc.org for it to play properly. Debian does not
include it for legal reasons. As far as I know, it has never been legally
evaluated, but it is probably illegal in a few dozen countries. I can't and
won't advise you to use it. If you want DVDs to play properly, you will
have to install it from source yourself, and take any legal ramifications.