As others have said, it's not dying. It's going mainstream. As that happens, the close culture the community has known will start to subside a bit, and there will be less need for people to seek out forums to get information. They can ask that guy or lady they know, or go down to the vape shop rather than posting about it. Second-hand sales might be moving to places like Craigslist (mainstream), but I can't be sure of that.
I've seen similar things happen in various hobbies of mine. It's the way it goes as something gets more popular. The forums will increasingly be a place for the most invested, hard core people to talk, but the mainstream and casual people will just do things offline.