This should be fixed in gstreamer-vaapi version 0.5.3, released 18 April 2013. It contains the patch "Fix fallback to sofware decoding if no hardware decoder is available". Also, gstreamer-vaapi version 0.3.7, released 26 June 2012, contained the patch "Fix vaapidecode to report unsupported codec profiles".
This should be fixed in gstreamer-vaapi version 0.5.3, released 18 April 2013. It contains the patch "Fix fallback to sofware decoding if no hardware decoder is available". Also, gstreamer-vaapi version 0.3.7, released 26 June 2012, contained the patch "Fix vaapidecode to report unsupported codec profiles".