My gut also tells me Alonso is going back to the Mets. The best angle I see is that Vientos can't play 3rd and if the Mets are horny to land Vlad, you can't have Vlad, Alonso and Vientos and one would have to go next year.
Bregman is the best positional fit, but I'd be shocked if he doesn't join the Tigers (what are those guys doing?).
A trade for Robert seems like the next logical pivot. I'd be pretty happy if we landed Robert as a 2nd big bat.