Uruguayan defender Britos headed his first Hornets goal to leave David Moyes' strugglers still seven points adrift of safety and with games running out.
Watford had been frustrated by Sunderland keeper Jordan Pickford, who made a string of fine saves to deny M'Baye Niang and Abdoulaye Doucoure.
But Pickford could not prevent Britos pouncing after a goalmouth scramble to condemn Sunderland to another defeat.
One goal is all you need to beat the Black Cats at the moment, as they have now not scored in their last five matches a nd not even Jermain Defoe, fresh from his England renaissance, looked likely to end their drought.
By contrast, a first win in five matches should ease Watford's fears of being dragged into trouble.
Pickford was quickly into the action for Sunderland, the youngster palming Niang's shot away before making a stunning, diving save to scoop Doucoure's follow-up over the crossbar.
Stefano Okaka should have put Watford ahead with a free header which he planted over, before Etienne Capoue curled narrowly wide.
Watford's injury curse struck again shortly before half-time when Younes Kaboul pulled up with a hamstring injury.
Read the full article on eurosport.co.uk: Sunderland in trouble as Britos fires Watford winner