Wattage I'm not as concerned about, I have dozens of appliances that use more than the 120w difference between the Vega and 1080. The monthly power cost difference is relatively minimal unless you are mining at 100% 24x7. What I'm more concerned with is heat, since that heats up all of the other components in my PC and even to a small degree the room temp. Heat also is going to likely hurt oc headroom, which has been an issue on recent AMD cards.
If the benches are accurate, I'd put a fair price on a RX Vega at $399. That would beat the 1070 on pure performance/price (I paid $399 for my 1070 new), but also offsets the higher power and other minor AMD cons. The NV platform is much more mature now having been out a year, oc's well, gets quicker driver updates than AMD, and will probably run cooler. Any more than $399 and I'd happily buy a 1070 for the same price, a 1080 in the low $500's, or go $700 for a much faster 1080ti. That's all excluding the mining price shenanigans which should eventually wind down.