Defoe finally broke through West Brom's stubborn resistance with a second-half winner at White Hart Lane.
The striker, recalled after a month spent in the treatment room and on the bench, swivelled and poked home in the 63rd minute after holding off the challenge of four Baggies defenders.
The goal came after a drab opening hour in which Tottenham looked anything but genuine title challengers.
Sloppy passing, wayward finishing, and a failure to carve open an organised West Brom defence meant Spurs were in danger of passing up another opportunity to put pressure on second-placed United and leaders Manchester City, before Defoe reminded manager Harry Redknapp of his killer instinct with a precise finish from 10 yards to ensure they started 2012 with a win.
Tottenham are now within striking distance of United and have a chance to confirm their status as contenders in their upcoming home games against Everton, Wolves and Wigan.
One troubling note for Redknapp will be his ever-increasing injury list.
Jake Livermore, William Gallas and Sandro all had to come off with injuries tonight and Spurs dearly missed the presence of Scott Parker and Ledley King, who were ruled out due to knee problems.
After seeing their rivals stutter over the festive period, Spurs had a golden chance to establish their position as London's top team and make serious inroads in to the grip the two Manchester sides hold on the league.
Injuries to Chris Brunt and Shane Long further boosted their hopes, but Parker's absence for the first time since he signed from West Ham left a gaping hole in Spurs' midfield.
Defoe, who has hinted he could seek to leave this month, looked keen to make a good impression, testing Ben Foster twice in the opening five minutes on a chilly night in north London.
Foster punched clear a teasing cross from Gareth Bale before Jerome Thomas hacked down the Welshman down to give Spurs a dangerous free-kick which Van der Vaart wasted by firing in to the wall.
The Dutchman quickly made amends by sneaking in front of his marker to clip a delicate shot with the outside of his boot which Foster tipped over at full stretch.
West Brom were forced in to a change when Gonzalo Jara came on for Thomas and Sandro then had to come off for Tottenham just before the half hour after pulling up with an injury. Livermore took his place.
Billy Jones put in a crucial block to deny Emmanuel Adebayor deep inside the Baggies' box after a one-two with Bale as Spurs continued to dominate.
West Brom boss Roy Hodgson cut a worried figure on the edge of his technical area, barely taking his seat during the opening half.
But despite their dominance, Spurs were failing to trouble Foster and organised defending from the visitors lead to an air of frustration inside the ground.
Van der Vaart drilled a low shot towards goal but Foster saved and Redknapp shook his head in disappointment at the half-time whistle.
Adebayor's clever backheel found Defoe inside the box just after the break but the striker shinned his effort way over.
He then flicked a deflected backheel of his own from Van der Vaart's cross that dribbled just wide.
Graham Dorrans and Jones went in to the book for two late challenges on Bale after failing to handle the winger's pace.
Craig Dawson flashed a header just wide in a rare West Brom attack before Spurs came close with a curling Luka Modric free-kick that sailed just wide.
Sloppy passing led to further frustration, but concerns the game was heading for a goalless draw ended in the 63rd minute when Defoe bagged his 10th of the season.
The ball ricocheted off Van der Vaart from Benoit Assou-Ekotto's cross and fell to Bale, who squared for Defoe. The England striker held off the challenge of the Baggies defence before turning and firing home with a low drive.
A full-hearted celebration followed at the relief of finally breaking the deadlock and the tension eased at White Hart Lane.
Tottenham's injury worries deepened soon after when Gallas hobbled off and was replaced by Sebastien Bassong.
Defoe, now full of confidence, had a goalbound shot deflected wide before Redknapp was forced in to another change.
A clash of heads between Younes Kaboul and Livermore left blood pouring from both players' heads and Livermore had to come off for Niko Kranjcar.
A poor tackle from the Croatian allowed Simon Cox some space on the edge of the box but the forward blazed over.
Brad Friedel then palmed away a strong effort from substitute Somen Tchoyi and Van der Vaart cleared desperately in the six-yard box as West Brom applied the pressure in search of an equaliser.
The match almost ended on a sour note as Bale charged angrily towards Simon Cox after being fouled by the Baggies player, but he was held back and Cox entered the book.