Thinking that it might be a matter of a waiting game until a friendship, romance or business has a breakup, then any former bitter confidantes might speak up.
imo.
Not to sound too pessimistic but I think something bigger than that needs to happen for someone to step forward and start talking. A lot of the shaddy characters that were behind Brianna's disappearance still operate in Vermont as we speak and the whole area there is very afraid of these guys. I was lucky enough to speak with a handful of people who clearly told me that they don't mind talking to me about the case, mostly because I'm not from Vermont which apparently changes the dynamics entirely.