Minnesota announced the move on Saturday. The Vikings waived fourth-year safety Antone Exum Jr. in a corresponding move.
Sloter entered the NFL in May as an undrafted rookie free agent signing by the Denver Broncos. He was released by Denver on Sept. 2 before signing with the Vikings' practice squad on Sept. 4.
Sloter completed 31-of-43 passes for 413 yards, three touchdowns and zero interceptions this preseason for the Broncos.
Bradford is questionable for the Vikings' game Sunday due to a knee injury. He was limited in practice this week but traveled with the team to Pittsburgh. Minnesota currently has Case Keenum on its depth chart behind Bradford as Teddy Bridgewater sits on the physically unable to perform list.
Keenum completed 196-of-322 passes for 2,201 yards, nine touchdowns and 11 interceptions in 10 games last season for the Los Angeles Rams. He posted a 4-5 record as a starter.
A source told ESPN that Bradford had an MRI on his left knee on Tuesday, which did not reveal specific damage. Bradford had ACL surgery on the same knee in 2013 and 2014.
Vikings coach Mike Zimmer was asked about his concern for Bradford on Friday.
"I'm concerned about all of our players," Zimmer told reporters.
Zimmer said he didn't know if Bradford will play but he does "anticipate that he will."