The Vega 56 manages around 31MH/s for Ethereum, and the Vega 64 does 33MH/s. Overclocking the VRAM helps boost both cards closer to 40MH/s right now. So at launch, that's not super promising.

http://www.pcgamer.com/the-amd-radeon-rx-vega-56-and-vega-64-review/