Vincej, 26, made his Major League debut with Cincinnati in 2017, appearing in nine games as a September callup. He went 1-for-9 with two runs scored after posting a .270/.325/.370 line with three home runs and 38 RBIs in 110 games at Triple-A Louisville.
The Mariners claimed utility infielder/outfielder Andrew Romine from the Tigers on Thursday and catcher David Freitas from the Braves last week. Their 40-man roster sits at 37.
Vincej is regarded as an outstanding defender and earned a Minor League Rawlings Gold Glove Award in 2016 when he led all Minor League shortstops with a .991 fielding percentage at Double-A Pensacola.
The San Diego native was a 37th-round Draft choice in 2012 out of Pepperdine, where he earned the Brooks Wallace Award as the best shortstop in college baseball. He's hit .272 with 270 runs and 46 stolen bases over 578 games in six Minor League seasons.
Vincej wasn't on the Reds' 40-man roster until being added for the last month after impressing the club with his offensive improvement over the past two years.
"He was giving very competitive at-bats and being a very solid guy," Reds manager Bryan Price told Cincinnati reporters at the time of his promotion. "He doesn't strike out much, he puts the ball in play. He can do a lot of things with the bat as far as situation play. He's a very, very steady shortstop."
The Mariners have four players on their 40-man roster capable of playing shortstop, with starter Jean Segura and utility infielder Taylor Motter joined by Romine and Vincej.
Greg Johns has covered the Mariners since 1997, and for MLB.com since 2011. Follow him on Twitter @GregJohnsMLB. This story was not subject to the approval of Major League Baseball or its clubs.