NorthridgeFix and northwestrepair are well regarded when it comes to GPU repairs, but they're both on the west coast. Quite a few electronics repair shops could handle some degree of GPU repairs, so it might be worth looking up local shops. However, any professional repair that involves anything beyond the basic is going to be expensive, mostly because of labor and insurance. A Vega 56 almost certainly isn't worth getting professionally repaired as the repair could easily cost more than the card is worth.
There is a huge spectrum of repair that could be needed. Your particular problem could be as simple as a corrupt firmware that just needs flashing, or it could be as profound as a dead GPU package that would need replacement. Quite a few people here would be able to handle less involved fixes; a fair number of us could diagnose and bypass or repair a busted power stage, but very few have the skills or tools to do a GPU package swap/reballing. Even if you found a hobbyist with the ability and inclination, they'd almost certainly charge a fair bit and be unable to offer you much in the way of a warranty.
