Given that on Radeon systems both APIs are available and, depending on the card, one works better than the other, an env variable that would allow to choose between them would be usseful.

I'm running an old HD 3650 GPU and VAAPI does not work properly; however VDPAU does. The problem is that Steam doesn't seem to offer some mechanism to prefer one over the other and it defaults to VAAPI.

VAAPI has high CPU usage and the console gets flooded with

libva info: VA-API version 0.40.0
libva info: va_getDriverName() returns 0
libva info: Trying to open /usr/lib32/dri/r600_drv_video.so
libva info: Found init function __vaDriverInit_0_40
libva info: va_openDriver() returns 0
Installing breakpad exception handler for appid(streaming_client)/version(1.0)
libva info: VA-API version 0.40.0
libva info: va_getDriverName() returns 0
libva info: Trying to open /usr/lib32/dri/r600_drv_video.so
libva info: Found init function __vaDriverInit_0_40
libva info: va_openDriver() returns 0