Vega uses the same technology as the current Polaris - except for the use of HBM memory instead of GDDR.
It's more the "next generation of Fury" than "the next generation of Polaris".
Think "Fury/FuryX/Nano" vs "R9 390/390x".
The RX 490 (if it actually appears) still looks more likely to be a "dual Polaris" card than anything else.
Vega is new arch (well modified more than Fury or Polaris) Its Single chip (there will be several versions - like Fury and Fury X, Nano? - but I'm not 100% sure)
Its likley 8Gb HBM2 but it is announced to work as cash for all memory in the system (it may have onboard SSD?) and we need to see how it will work for mining in terms of latency and other practical aspects of memory (might be similar to furys HBM and GDDR5X) - so it might be good or not so good for mining..
In terms of the TDP - its likely to be 250W card (top card but there might be lower TDP version).
It should be expensive and comparable in perf with Titan XP (but that is yet to be seen) The big question is when it is released and quantities (expected to be really poor if it launches feb-march - less than 10000 units world wide) Should OC poorly due to HBM2