Freesync is about the only reason to go Vega, those are much more reasonably priced than their gsync rivals. The miners are still piling up the vegas for now, Amazon, Newegg ran out of stock in minutes, and they've been marked up, don't think you'll be getting one for 400$. Apparently there's some attractively bundles for vega 56/64 with a freesync monitor and some games, but you already have the monitor.
Spend a bit more and go 1080ti maybe ? A single card is more than enough to crush mostly anything at 4k.