I'll put out a hot take and say Luepke will make the 53 over Vaughn.
Vaughn's not a special teamer. He can't block on ST, can't play gunner, is too slow for KR, and never fielded a college punt. He does have a lot of potential as a punt returner, but I have never heard of a developmental ST prospect.
His way onto the field is as a pure running back. Pollard's locked in, Davis is locked in because the team will want one bigger backup RB, and then it's down to Vaughn and Jones for the final RB spot. And I know I've said it before, but Vaughn is a sub-threshold athlete as a pure RB.
Luepke though, brings an FB skillset that's rarely utilized but is unique. And he's an easy projection to ST, because he's athletic and physical enough to run down returns and has lots of prior blocking experience. For a UDFA, he's got a pretty direct route to making the final roster.