Chris Iwelumo scored an extra-time winner to deny battling Wycombe who more than matched their Championship opponents for 120 minutes.
Blues should have been awarded a penalty with time running out when Grant Basey’s cross hit Jonathan Hogg’s hand, but Wanderers were denied a spot-kick and slipped to an underserved defeat.
The hosts should have taken the lead in the second minute but Jonathan Hogg somehow headed Carl Dickinson’s cross wide from five yards.
Richard Logan was inches away from scoring his first competitive goal for the Blues 60 seconds later when he reacted quickest to Joel Grant’s initial effort, but Manuel Almunia got just enough on the ball to divert it wide.
Logan then headed home the resulting corner, but the striker was adjudged to have fouled his marker.
Wanderers were the better side for large periods of the first half and didn’t let their Championship opponents have any time on the ball.
That pressing was a feature throughout the match and Watford didn’t have another shot on target until the 44th minute when Mark Yeates cut in from the right and fired straight at Bull from 25 yards.
Anthony Stewart had the final chance of the first half and his looping header from Matt Spring’s free-kick drifted just over the bar.
Yeates came close with an audacious curling effort two minutes after the restart as the hosts started the second half brightly. Joe Garner then fired a half-volley straight at Bull, but the Hornets were still struggling to create clear-cut chances.
Logan had a chance to give the Blues the lead in the 73rd minute after Spring’s exquisite through ball played him in on goal, however, the striker sliced his shot wide when he should have done better.
Stuart Lewis, who was superb throughout, drilled a low strike just wide of the left-hand post with Almunia well beaten.
Wanderers almost grabbed a fortuitous winner with the game entering injury-time when the ball bounced off Spring and had to be tipped over by a backpedalling Almunia.
Chances were few and far between in the first half of extra-time, but the hosts made the breakthrough when Matej Vydra broke down the right, cut the ball back and Iwelumo bundled the ball in from close-range.
Blues should have been awarded a penalty when Grant Basey’s cross was clearly handled by Hogg, only for the appeals to be turned down by referee Carl Berry.
Wycombe: Bull, McCoy (Johnson 22), Dunne (Basey), Doherty, Stewart, Spring, Lewis, Bloomfield, Grant, Wood, Logan.
Subs not used: Ingram, Ainsworth, Kewley-Graham, Taylor.
Watford: Almunia, Doyley (Hodson 70), Dickinson (Forsyth 76), Taylor, Nosworthy, Hogg, Eustace, Abdi, Yeates, Vydra, Garner (Iwelumo).
Subs: Bond, Buaben, Anya, Thompson.
Attendance: 5,343 (658 Wycombe)