Nintendork :
Vega56 is still cheaper and it's still the better value overall for newer games that are basically AMD optimized.
And what games would those be?
Battlefield 1 - nope
Sniper Elite 4 - nope
Ashes Of Singularity - nope
Metro: Last Light Redux - nope
Middle-earth: Shadow of War - nope
Rise of the Tomb Raider - nope
Tom Clancy’s Ghost Recon Wildlands - nope
Tom Clancy’s The Division - nope
Warhammer 40,000: Dawn of War III - nope
The Witcher 3 - nope
Hitman 2016 - nope
Resident Evil 7 - nope
Doom (Vulkan) - nope
Prey - nope
GTA V - nope
Formula 1 2017 - nope
Project Cars/Project Cars 2 - nope
Assetto Corsa - nope
DiRT Rally/DiRT 4 - nope
Euro Truck Simulator 2/American Truck Simulator - nope
Now what games do the RX 56 beat the 1070 Ti by a few FPS meaning slightly better optimized for AMD? Destiny 2 and Dues Ex. That's it.
Regarding value, sellers are overpricing the 1070 Ti's right now. But between both cards, prices fluctuate wildly depending on where you shop. In the US for example, Amazon has an XFX RX 56 variant for $565, but a Sapphire one for $499. They have a GTX 1070 Ti EVGA FTW2 model for $499, an MSI 1070 Ti for $529, and an ASUS ROG Strix Gaming 1070 Ti for $550. NewEgg is selling that same XFX model RX 56 (model #VEGMLBFX6) for $419 and ASUS ROG Strix Gaming model 1070 TI for $495. You can get an ASUS ROG Strix gaming GTX 1080 for $529 on Amazon whereas they are charging $600 for an XFX RX 64.
Finally, the only Vegas I've seen, either 56 or 64, only have the lame blower reference cooler design meaning little overclock headroom (on top of Catalyst drivers not playing nice with Vega overclocking). When you take a 1080 Ti through overclocking paces, it leaves the RX 56 behind and puts it square into reference GTX 1080 performance. So the point is, it boils down to where you find either GPU cheaper. Right now, the 1070 Ti's are overpriced and vendors are price gouging them into GTX 1080 territory - real world 1070 Ti prices are not where they need to be (lower and closer to the $430-$450 mark).