If you have a decent power supply, and insist on AMD, then it is possible to find Vega 56 cards in the same price range as RX 580 and 590 cards. There are still a lot of Polaris cards coming on the market ever since Etherium mining crashed, so you're taking a risk that you'll be buying a card froma mining farm. Vega's initial higher price, and power consumption, made it less profitable to use for mining.
I think the biggest risk with Vega 56, is the likelihood of a bad flash rendering the faster switch setting unusable. There are so many horror stories in tech forums where people bricked the high power switch, and weren't able to recover it, that I've seen. I picked one up, cheap, with a bad flash, but I bought it to use in a Mac Pro, so the low power switch was always going to be the only one that I was ever going to use, so I didn't care.
Vega is also not going to be on the latest drivers, the same as Polaris, though, and Vega is broken in games that use mesh rendering, like Alan Wake 2. You'll probably want at least something from the RDNA2 generation, for the latest games.