I think DL would prefer to codeshare with VX rather than buy them. Honestly, wouldn't be surprised if DL tried to get VX to replace Alaska once Alaska leaves the DL agreement (it will eventually happen if DL keeps building up Seattle). VX's customers love the airline for its amenities and superior product, and could easily leave and head to UA if DL took over.
I think B6 would most likely buy VX. B6 is very big in the Northeast and Florida/Caribbean. They are pretty small outside those areas except for a small base at LGB. They are also run in similar ways, have similar groups of loyal customers, and operate the same type of aircraft (B6 has A320s, A321s, and E190s and VX has A319s, A320s, and soon A321s).