having read the reviews that barefeats did with Vega cards in eGPU boxes the key will be drivers. currently there no good drivers for Vega on High Sierra. One would hope that Apple is merely just sitting on them until the iMac Pro releases, if not it will be a big disappointment
http://barefeats.com/early_vega.html
That article is old. The drivers are fine under 10.13.2. I run a Vega 56 as an egpu. It works great, the only issue is the loud fan.