A return of one point from four matches had left Walter Mazzarri's side in danger of being dragged into the fight for survival, but Saturday's deserved win kept them seven points clear of the bottom three.
Sunderland were lacklustre and the seven-point gap to safety looks like a mountain that David Moyes' men are ill-equipped to scale.
He was left exposed all too often and could not be blamed for the decisive goal, which came from a back-post header after Stefano Okaka had struck the crossbar in the 59th minute.
Reports this week have suggested Moyes will remain in the job regardless of whether he keeps Sunderland in the top flight, but with just one win in 2017, Championship football beckons for the Black Cats.
Heurelho Gomes' scrambled to tip Adnan Januzaj's deflected cross wide early on, but Sunderland would have to wait until the final five minutes of the half for their next attempt at goal.
It was Watford who forced the issue and Pickford twice came to his side's rescue in the 11th minute, denying M'Baye Niang and then Abdoulaye Doucoure in quick succession.
Craig Cathcart's searching cross also caused problems for the visitors, but Doucoure could not direct his header on target.
The Hornets were guilty of playing one too many passes on the edge of their opponents' box, with Doucoure turning down the chance to shoot when well placed 20 yards out.
But Mazzarri's afternoon took a turn for the worse when former Sunderland defender Younes Kaboul went down injured, having stretched to reach a loose ball in midfield.
Januzaj almost rubbed salt in the wound as his long-range effort clipped off Cathcart and squirmed wide of Gomes' right-hand upright.
There was still time in the half for Niang to bring a smart near-post save from Pickford.
Defoe briefly went off after going down clutching his right leg early in the second half, but Sunderland's talisman was soon moving freely again.
John O'Shea made an important near-post interception from Britos' delivery, with Watford remaining in the ascendancy.
And the deadlock was broken when Okaka's header was turned onto the crossbar by Fabio Borini and Cathcart kept the ball alive for Britos to nod in from close range.
Substitute Isaac Success was thwarted by the impressive Pickford a couple of minutes later, while Januzaj shot tamely at Gomes at the other end in a rare opening for the away side.
Borini's low strike called Gomes into action late on, but there was to be no salvation for Sunderland, who travel to in-form champions Leicester City on Tuesday.
Key Opta stats:
- Miguel Britos scored his first Premier League goal in what was his 48th Premier League appearance.
- Sunderland have now gone 495 minutes without scoring in the Premier League.
- The Black Cats have won just one of their last 12 Premier League fixtures (D3 L8).
- Only Middlesbrough (4) have won fewer Premier League points in 2017 than Sunderland (6).
- Sunderland have kept just one clean sheet in their last 16 Premier League games away from home.
- By contrast, all of Watford’s clean sheets at Vicarage Road this season have come against the teams currently sitting in the relegation zone (Hull, Middlesbrough and Sunderland).