Gilles,
I have a similar problem (I have a NVidia card though). If I add a 3rd party plugin using cuda technology then hit play, I can’t render using GPU (Same undertermined error as you). If I close Vegas then restart it, Don’t play, and render immediately, I can use GPU with Mainconcept. I have an open case with sony for months… no solution so far except the workaround above (or use CPU only but render a very very long…)