Some posts in this thread are conflating two clearly separate things:
1. The final date for a turn-in, which is 12/31/2018, and which VW is required to comply with per legal agreement.
2. Dealers participating as "hosts" in the buyback program. Nothing in the legal agreement requires any VW dealer (which are not owned by VW, but are all independently owned and operated) to participate, or, once participating, states when or if they can end such participation.
It is not completely clear in the OP which of these things is being discussed, but my opinion is that it leans rather strongly in the direction of #2. I think anyone who believes their dealer of choice is about to pull out of the program should consider pulling the trigger ASAP and turn the car in. I have no particular dealer in mind. Will they all pull out? Will I have to drive to Herndon, VA and turn it in? We live in interesting times....