Do you mean as an encoding card? If so, yes you should be able to. Heck I've used an AMD card with a cheap NVidia GT 720 to encode video. The hard part is making sure that your software sees and uses the correct card. In OBS you have to make sure you're using the advanced view mode to change the video adapter used for encoding.
YES you can
run your game on your Vega and Capture the game with OBS Studio + AMF Encoder Plugin
you can choose your video adapter in "Advanced" View-Mode in Encoder Tab
ps i tested it with my RX580 4G + RX460 2G (low profile)