Given that AMD's partner SK Hynix will be in HBM2 production by Q3 2016, a mid-late Q4 2016 launch of Vega is possible.
http://hexus.net/tech/news/ram/91100-sk-hynix-schedules-4gb-hbm2-mass-production-q3-year/
The thing is if Polaris 10 is 232 sq mm with 2560 sp / 4 shader engines / 64 ROPs then Vega 10 can definitely double it 5120 sp/ 8 shader engines / 128 ROPs and still come in at 450-500 sq mm on GF 14LPP. Hawaii basically doubled pitcairn in all resources(slightly more than 2x on sp)
With a 4096 bit HBM2 running at 1.6 - 2.0 Ghz speeds we are talking of 820 - 1024 GB/s . Thats a massive amount of bandwidth.
Since AMD has already designed a new memory controller with improved L2 cache and memory compression in Polaris we can expect something which is as good or even better. Hitting the peak holiday season helps maximize sales opportunity, so a Nov/Dec 2016 launch is a good bet. I think Vega 10 will be a 4K monster.