Bent had made 16 appearances without a goal since his summer switch from Charlton but has now hit the target twice in four games after his 18th minute strike.
Second-placed Birmingham had to secure their fourth win in five games the hard way as they fell behind after just six minutes to a close-range finish from Tamas Priskin.
But Birmingham were back on level terms immediately through a 10th goal of the campaign from Kevin Phillips before Bent struck.
Substitute Cameron Jerome then looked to have made the game safe five minutes from time with his second goal in two games.
But Ross Jenkins' last minute strike meant that Birmingham suffered a nervy four minutes of stoppage time.
It was Watford's fourth game under the control of Brendan Rodgers and the former Chelsea coach is still looking for his first win.
The defeat at Birmingham ended a bad week for Watford coming hard on the heels of being knocked out of the Carling Cup by Tottenham.
But Rodgers can take heart from a storming second-half display that was worth a point.
Watford initially stunned Birmingham after Priskin found himself unmarked in acres of space just six yards out.
His first effort was pushed out by Maik Taylor but the rebound fell straight to the Hungary international who had the simple task of stabbing the ball into an empty net for his fourth goal of the season.
It may have been the perfect start for Watford, but scoring so early was perhaps the worst thing they could have done as it sparked Birmingham into life.
Watford's advantage lasted barely 60 seconds before Phillips rose to head home a right wing cross from Nicky Hunt for his 10th goal of the season.
Birmingham had Watford on the ropes at this stage and their pressure again paid after the lively Quincy Owusu-Abeyie lost the ball on the edge of the area.
The loose ball fell invitingly to Bent who rifled a low 20-yard drive past Scott Loach.
Phillips and Quincy then missed chances to put the result beyond doubt before half-time and failing to take those opportunities handed Watford a lifeline they did their best to grasp.
Jenkins was denied an equaliser in the 53rd minute by a last-ditch block from Radhi Jaidi before the woodwork frustrated the Vicarage Road side.
Leigh Bromby let fly with a 25-yard free-kick that beat Taylor only to bounce off the top of the bar.
Taylor then rescued Birmingham after 75 minutes by stopping Jaidi from turning the ball into his own net after a right-wing cross from Jobi McAnuff, before John O'Toole just failed to get on the end of a cross at the far post.
Watford were made to pay for not making their pressure pay when Jerome weaved his way into the area and beat Loach with a curling drive.
That should have ended Watford's challenge once and for all but Birmingham had to endure a nervy finale after Jenkins won a battle of pinball in the area to beat Taylor from six yards.