Vata scored a stunning goal in the 88th minute, just after Hull substitute Chris Bedia's messy shot seemed to secure a debut victory for former Reading manager Selles.
The Tigers halted a six-game losing run in the Sky Bet Championship, but they remain bottom of the table and without a win since October 1.
Watford extended their unbeaten league run to five matches, but the point did little for their play-off hopes as they slipped to eighth.
Hull began with a spring in their step after Selles' recent arrival and Ryan Longman wasted an early sight of goal after being played in by Kasey Palmer.
TrendingBut it took another 30 minutes before Hull had their first effort on target, with Ryan Giles' low effort easily gathered by Watford goalkeeper Daniel Bachmann.
Watford had not troubled home goalkeeper Ivor Pandur either and the game's first spark was provided by an off-the-ball push-and-shove between Hornets midfielder Imran Louza and Palmer in the 36th minute.
Both players were booked and Hull were briefly stirred into action as Regan Slater's cutback in the area picked out Joao Pedro, whose first-time shot was well-blocked by Watford centre-half Ryan Porteous.
Watford, and Giorgi Chakvetadze in particular, stepped it up at the start of the second half.
The Georgia forward's ball carries pushed the Tigers deep and after jinking his way into space on the edge of the area, he curled a right-footed effort narrowly off target.
Slater then made in-roads into the opposite box and his low cross fizzed behind Longman before Pedro further lifted the Tigers' fans by racing clear on to Palmer's pass and firing a low angled drive wide.
Hull's confidence grew. Longman headed skipper Lewie Coyle's cross straight at Bachmann and substitute Mason Burstow, with his first touch, forced the Hornets goalkeeper into a crucial low save.
The home side thought they had snatched a crucial win in the 82nd minute when substitute Marvin Mehlem's sliced effort deflected in off Bedia, possibly via his hand.
But with the clock ticking down, Watford replacement Kwadwo Baah cut the ball back from the right and Vata swept home a first-time finish.
Hull's Ruben Selles:
To follow...
Watford's Tom Cleverley:
To follow...