Jackson became the eighth draftee to sign when he finalized his deal Thursday. Jackson was taken 81st overall out of Mississippi State and is expected to compete for a starting job.
Gilreath played three games with Pittsburgh in 2012 and set Big Ten records for kickoff returns (135) and yards returning (3,025) at Wisconsin.
Vaughn spent time last season on the New York Jets practice squad.
Oakland waived receivers Jared Green and Noel Grigsby and punter Daniel Zychlinski.