If NF were also coming to an end (and I don't know if that's the case), it would probably be PAD's decision or that of his editor (and I believe the NF books have mostly been edited by Ed Schlesinger), so it'd be totally independent of the Vanguard decision.
If it comes to an end with BMB it's certainly not PAD's decision. From his blog (May 21st):
Why possibly last? Because the economy sucks, the publishing industry is slowly disintegrating, and rather than make long term plans, Pocket is making decisions entirely based upon what’s happening right now. So they’re waiting to see what the sales are on this book before deciding on whether there will be any more.