I don't know what would happen with the Vram. Probably nothing I guess. 12gb for all of them. Maybe they can increase the Titan black to 16gb.
I don't know if 12gb is the max for these cards or what. 16gb for titan black and 12gb 1080ti would make some sense. Full chip Titan black, existing cut down version or similar for 1080ti.
Yes, it could go one way or the other. Since GDDR5X supports non-power-of-2 chip capacities, they might end up with some interesting capacities, that is if they decide to use the same 384-bit wide memory on both.
A: 1080 Ti: 9 GB, Titan X Black: 12 GB
B: 1080 Ti: 12 GB, Titan X Black: 18 GB