Will Buckley's 90th-minute winner sunk Leicester to keep up Brighton's chase of the play-off spots in a match that saw three players sent off.
The visitors' Jermaine Beckford controversially saw red after an exchange of words with Lewis Dunk.
Matt Sparrow's second yellow in the 85th minute evened up the sides before Buckley volleyed home Vicente's cross.
Brighton could have had another but Neil Danns hauled down Vicente to earn the third red card of the contest.
The hosts are still unbeaten in 2012, a run of eight matches that includes an FA Cup victory over Newcastle, and now lie a point and four places off the final play-off position.
Leicester had the best chance of the first half after a long ball forward caught out the Brighton rearguard, but Beckford's weak sidefoot effort was smothered by Peter Brezovan.
The Foxes striker's afternoon came to an early end, to the disbelief of some of his team-mates, after the linesman drew the referee's attentions to an apparent comment he made to Dunk after mistiming a challenge on the Brighton defender.
Brighton pressed forward with a man advantage and Buckley should have done better than to shoot straight at Kasper Schmeichel when put clean through.
The game appeared to be heading for a stalemate but, after Sparrow had been dismissed in the 85th minute for a reckless lunge on Leicester substitute Ben Marshall, Buckley applied a scoring touch to Vicente's teasing cross to wrap up three points.