I actually did this math last night while talking about the inflated Vega prices.

If you're only talking two 570s, a single Vega 56 is going to give you more hashrate with less (or equal) cost, and less power draw. Three 570s has its benefits, but you're just making your power company happy.

My numbers may not be representative of anyone who has really dialed in their settings, but think of having to do that 3 times more than a single Vega. I am taking into account the average person's experience.