Vega has better hashrate than 2 580s.
The most important thing people often keep omitting from posts like this is that Vega is
ONLY faster than two 580s on cryptonight. This is not the case for every other algo out there. The most popular of which is (and has been for more than a year now) ETH/Dagger. Two 580s will do close to 60 MH/s while one Vega will be doing up to 45 MH/s.
Cryptonight profits might (and probably will) go down at some point. It has happened before, there were periods when XMR was the most profitable coin for AMD cards, but then it would go down and ETH was pretty much the only option available for months. Suggestion to go with single Vega against two RX 580s just because cryptonight has been so profitable lately is pretty shortsighted in my opinion.