I doubt anyone here has experience with the new Vega cards from AMD. I use a Fury X and aside from a few plugins that don't work, I have no issues with it. There where already problems reported with the RX580 and it seems the Vega cards are also having issues. VP14 still uses the same old Sony Vegas GPU acceleration code that has been developed for Sony Vegas Pro 11 and never been updated.
We can only hope VP15 will be better but I wouldn't hold my breath.