How, I haven't heard of Sony Vegas for a while, I used to use that.
Back to topic. Let me see if I can help you out.
Video editors can be picky with the drivers, so I would advise you to use WHQL drivers, DaVinci won't work for me properly if I use a non-WHQL driver at all. It just won't see my GPU and thus not allowing OpenCL or GPU acceleration. It happened with my RX480 and RX6800.
Make sure you enable the GPU acceleration inside the application and you have version 15 or later.
If you can't see or select the card, then the driver advice is looking a go-to right now.
Also, Sony does not support Vegas anymore, they sold it to Magix, maybe have a look into their forum?
https://www.vegascreativesoftware.info/us/vegas-pro-forum/
Good Luck
The Englishman