I don't know. It was only a few days ago I posted that I had a feeling he was being softened up and that he appeared to be telling the truth when he said he loved Eileen.
If that is true (as I think seems likely now), then him doing the dirty on Vinny seems possible. It could be Vinny he's putting on an act for when they're planning to do a runner together, not Todd when he says he's staying. Or perhaps Vinny will try to kill Eileen and Phelan will kill him to save her. Both scenarios are plausible to my mind.
Or there could be a completely unexpected twist in the tale...