Vega sucks for ETH mining. The only thing it wins in is density. Underclocked and undervolted RX 480s have been making bank since June 2016, and do so while staying cool and quiet. Not to mention I paid less for a pair of them than a single Vega.
I know its a bit late for the mining craze.
But I think the new Intel CPU with Radeon and HBM2 will do quite well on Ethereum mining. I mean, it has the bandwidth to do so, and its Polaris-based.
The top end is a 100W TDP part with 1536SPs, ~1.1GHz clock and HBM2 memory with 200GB/s bandwidth. The bandwidth is equivalent to 6.4GHz on Polaris 10 parts. Maybe 18MH/s base.
I assume the 100W TDP is basically equivalent to a 35W H-class CPU with 65W left for HBM2 and GPU. That leaves 60W for GPU and 5W for HBM2. Ether mining has the CPU at idle so the GPU and HBM2 has even more to work with. I'd think it'd do it under 70W though.
The efficiency is gained a bit because its probably a binned part for mobile, but the real gain is not needing extra PCB on a discrete Polaris part along with HBM2 being very efficient on a bandwidth to watt basis compared to GDDR5.