I upgraded to 5.15.96. It did change the output of vainfo, but seemingly for the worse. Now it just outputs:
libva info: VA-API version 1.7.0
libva error: vaGetDriverNameByIndex() failed with unknown libva error, driver_name = (null)
vaInitialize failed with error code -1 (unknown libva error),exit
I may try 6.0.9, which this person says is stable with Zorin.