After doing some research, I've just found useful blog for your mining. Hope it may help: http://cryptomining-blog.com/tag/cryptonight/

http://cryptomining-blog.com/9314-quick-guide-on-using-amd-radeon-rx-vega-gpus-for-cryptonight-mining/

The two choices for best performance at the moment are Cast XMR and the latest version of XMR-stak. If using XMR-stak just make sure you configure the miner with two threads per GPU, this would require more virtual memory.