Vaccaro, 27, worked out for the Titans on Friday after starting safety Johnathan Cyprien was diagnosed with a torn ACL on Thursday.
Vaccaro had gone unsigned this offseason along with several other veteran safeties, as the market for their services didn't develop as expected.
The Titans also agreed to a deal with safety Jason Thompson.
The Titans tried to bring in Eric Reid for a visit, but he had "multiple flight cancellations," sources tell ESPN's Adam Schefter.
Reid had three separate flights from New Jersey to Tennessee canceled, sources told Schefter. The Titans are now set at safety, but the team is open to bringing Reid in again if the need arises, the sources said.
Reid has gone unsigned this offseason, and many believe it's because he has knelt during the national anthem the past few seasons to protest racial injustice. He has filed a collusion grievance against the NFL.
Vaccaro has shown Pro Bowl-caliber potential throughout his five-year career, during which he tallied 67 starts, 385 tackles, 8 interceptions, 7.5 sacks and 4 forced fumbles.
The 6-foot, 214-pounder, who was drafted with the 15th overall pick out of Texas in 2013, probably fits best as a strong safety playing closer to the line of scrimmage, but he has also often been used as a nickel back covering slot receivers. He also has been used as a pseudo-linebacker in certain packages.
Vaccaro finished third in the NFL Defensive Rookie of the Year voting in 2013, when former coordinator Rob Ryan moved him all over the defense as a versatile chess piece.
Vaccaro's past two seasons were two of his best, with a total of five interceptions in 23 games. But he missed seven games (including the playoffs) because of a lingering groin injury that ultimately required surgery in 2017. He also missed the final four games of 2016 because of an Adderall suspension.
ESPN's Mike Triplett contributed to this report.