Even with all the tweaking the most I've heard people get is around 38-40 MH/s with a substantial power draw. Given the price of these cards to buy I'm not convinced that it's worthwhile buying Vega cards for mining at this time. Unless the price for the cards drop... a lot. I can buy two RX 580 8GB for about the same price of one Vega 64 and get more hashrate for similar power usage.
Check out this review here for Vega cards versus RX 580s for hashrate and power usage.
http://www.legitreviews.com/amd-radeon-rx-vega-64-vega-56-ethereum-mining-performance_197049
New Blockchain Compute Drivers
Those still running old drivers on AMD RX series cards are leaving a heap of hashrate on the table, get yourselves the new AMD Blockchain drivers and give your cards a free boost in hashrate today!
http://www.legitreviews.com/amds-new-mining-block-chain-optimized-driver-tested_197095
You can grab the blockchain compute drivers here
http://www.legitreviews.com/amds-new-mining-block-chain-optimized-driver-tested_197095