To use vdpau you have to have mplayer compiled with vdpau video output:
Your machine have mplayer compiled without vdpau output:
[az@server Downloads]$ mplayer -vo help | grep vdp
Having just vdpau accelerated codecs is not enough. You have to have vdpau output to close the decoding chain by displaying decoded movie.
My mplayer with vdpau compiled in output:
zbyszek@desktop $ mplayer -vo help | grep vdp
vdpau VDPAU with X11
You use: Scientific Linux release 6.0 (Carbon) - this is RHEL6 clone designed for server purposes/scientific workstation. This is probably the reason why mplayer was build without vdpau. Maybe using any external repository with vdpau mplayer will help.
Try any popular desktop distribution - ask people here (better in new topic) what distros have mplayer with vdpau out of the box.