Harry Redknapp was made to endure a nervy night on his return to the Spurs dugout as his team edged into the last 16.
Redknapp, back on the touchline after spending his week contesting two charges of tax evasion, made the journey across London from Southwark Crown Court and watched his team put on a poor display in the first half, until Van der Vaart put the Premier League title chasers ahead with a 30-yard drive just before the break.
Championship side Watford, 35 places below Spurs in the league ladder, rattled the Tottenham woodwork on two occasions and Carlo Cudicini performed well to keep out lively striker Marvin Sordell in the second half as Spurs were made to work hard for their place in the next round.
Redknapp would have wanted an easier test on his return to action, but his team are safely through after avoiding an upset that looked likely for large parts of the game.
The 64-year-old Redknapp smiled as he assumed his place on the bench at the start of the game, clearly glad to be back at work following his week spent in court answering two charges of cheating the public purse.
Redknapp's assistant Kevin Bond insisted on Thursday that the absence of the Tottenham boss, who denies both charges in a trial which is expected to end next week, had not had an impact on the club's preparations for Friday night's game.
A starting XI that contained the likes of Luka Modric, Van der Vaart and Scott Parker were made to struggle in the opening stages, however, as Watford, clearly determined to cause an upset, started well.
With five minutes gone, Joe Garner released Sordell after a slip by Younes Kaboul but the pacy striker could only shoot at Cudicini.
The Italian then spilled Sean Murray's effort after a rare Modric error but Kyle Walker hooked the ball clear.
Troy Deeney fired a low hard ball across the box but Sordell put his shot wide.
Prince Buaben then belted a fierce drive just wide of Cudicini's goal in the 16th minute.
Almost half an hour had passed before Spurs had their first attempt on goal - a mis-timed volley from Jermain Defoe which Scott Loach saved comfortably.
Redknapp, who was being subjected to many taunts from the home fans, berated his team for their poor performance from the touchline.
Modric and Parker both gave the ball away in promising positions and Michael Dawson blocked Sordell's shot before Buaben fired wide from 18 yards at the other end.
His worries were eased three minutes from the break, however, when Van der Vaart picked up Walker's pass, advanced at the Watford defence, who backed off, and fired home from 30 yards past Loach, who will be disappointed he did not save the shot.
Redknapp brought on Aaron Lennon for Modric at half-time but Watford started the second period brightly and were nearly level four minutes in when Murray's curling effort was brilliantly tipped onto the post by Cudicini before Deeney blazed wastefully over on the rebound.
Tottenham rallied and Van der Vaart came close to making it 2-0 with a swirling free-kick before the Dutchman then struck the bar with a peach of a shot.
Mark Yeates, who was released by Tottenham five years ago, came on for Murray moments later.
The winger almost set up the equaliser within seconds, swinging in a deep corner that found John Eustace in the box but his header hit a post and went wide.
Bauben then latched on to a loose ball and fired a drive which was saved by Cudicini.
The Hornets almost pulled level with 10 minutes left when Cudicini saved Nyron Nosworthy's shot with his feet. The ball bounced out to Sordell but Walker flung himself at the ball for a crucial block.
Dawson denied Yeates a goal against his former club nine minutes from time with a block.
Watford threw everything at the visitors in the final 10 minutes but resolute defending ensured their hopes of earning a replay were dashed.