The Dutch left-back struck for the third time this season in the 49th minute at the Stadium of Light as David Moyes's men returned to winning ways.
The Dutch left-back struck for the third time this season in the 49th minute at the Stadium of Light as David Moyes's men returned to winning ways after back-to-back losses to Swansea City and Chelsea.
With 21 of the last 24 teams who were bottom of the table on Christmas Day having been relegated at the end of the season, it was a timely moment for Sunderland to abandon the basement position.
They moved up to 18th place, just a point from safety, while Watford slip one place to 12th after a fourth successive away defeat.
In a misleading preface to what would follow in a drab first half, both goalkeepers were tested inside the first two minutes.
Sunderland defender Papy Djilobodji saw a header fielded by Watford's Heurelho Gomes before Jordan Pickford was forced into a finger-tip save by Nordin Amrabat's volley at the other end.
Watford captain Troy Deeney remains a goal short of 100 for the club and he spurned a chance to reach his century when he opted to pass to Odion Ighalo, allowing Sunderland to clear.
Sunderland striker Jermain Defoe then saw a shot blocked by Miguel Britos, who headed wide of the hosts' goal following a free-kick shortly after.
Watford were holding their own, but manager Walter Mazzarri was forced into a defensive reshuffle in the 31st minute when Sebastian Prodl had to cede his place to Christian Kabasele.
It was not until stoppage time that the first half yielded a chance worthy of the name, as Van Aanholt raced through, only to be thwarted by Gomes.
The former Chelsea player would not be denied, however, and duly broke the deadlock four minutes into the second half, swivelling to score via the post from Defoe's pass.
The goal emboldened Moyes's players and the hosts went close to adding a second goal when Victor Anichebe crashed a shot narrowly wide.
Deeney had a sight of goal in the 72nd minute, but he headed wide from Amrabat's cross, while Pickford saved from Daryl Janmaat and Ighalo amid late, but fruitless, Watford pressure.