Sadly the AMD Blockchain Compute driver hasn't been released for Linux yet.. so mining with RX Vega and Simplemining OS wouldn't be optimal atm as far as I see it.

http://support.amd.com/en-us/kb-articles/Pages/Radeon-Software-Crimson-ReLive-Edition-Beta-for-Blockchain-Compute-Release-Notes.aspx

I have no clue if Simplemining OS even supports RX Vega, just thought I should mention the lack of proper driver for Linux