I would see it as a worthwhile upgrade, as long as you have games or programs you want to be able to run.
Im currently thinking of doing the same myself due to more recent games starting to lag behind a little and crossfire support at times being poor. In my case a 1080 would be worthwhile due to having performance similar to if both my cards were working with better pacing.
I would look at Vega prices compared to gtx 10 70's and 80's and go with whatever is cheapest, performance is quite close depending on which programs you use as benchies.
TLDR: Worth it if you want more performance
and choose whichever brand is cheaper.