Keene is 6'4" 253 pounds... he is a tightend, the Patriots desperately need TE's (plural). I get that Virginia Tech used him in offbeat ways but that was more an extension of their offense, they are an option offense or a QB running offense that doesn't use a lot of traditional vertical routes. And BB specifically told Keene he needed to make adjustments to play TE for the Pats when they spoke.
Johnson looked decent blocking, but he only made the team because they had a practice squad exemption with him being an international player. He also only played for a month and got hurt, and FB is a notoriously tough position, a high impact position.
Vitale is a halfback with FB strength, a really good athlete, and he was one of the first free agents they went after. I'd say he's as close to a lock as possible assuming health. If we have good physical TE's blocking on the edges along with big physical WR's, you'll want a fast physical FB because all of our RB's are fast and quick but undersized, none of them will be confused with Corey Dillon or LeGarrette Blount. The quicker you can get to the hole and block it the better. I loved the Vitale signing.