You do not metion your player. You might need patches, or VAAPI aware codecs to get full video acceleration. Also be aware that doing HD decode takes a LOT of graphics memory, so your system will need to have available RAM for the graphics driver to use to do a proper decode with VAAPI.
You should use the VA API library that is available for your distribution.
Also, have you read the white papers here on EDC about video decode and how to tune it for your use?
Hope this helps.