I don't have a Vega nor have I even tried Crossfire in a couple generations. I have heard many here ask this question and most are told that it just doesn't work right or isn't worth the effort anymore. If you can get it working to your satisfaction, I do wish you luck. Most find that only a few games utilize it in any real capacity and it causes issues then with most the remainder.
i found an article from the last year you might find helpful: