Vega has better hashrate than 2 580s. Vega also has lower power consumption than 2 580s. The only thing I see as advantage of 580 is when a card fails, you will still be able to mine with the other 1. Also I don't know the availability between 2 gpus but profit, price and power consumption in consideration, choose Vega.
Thank you your reason for the RX 580 speaks makes sense, I had already considered. The only question is how it behaves when I underclock all cards ..I usually underclock them to about 800Mhz. for example 800/1500