The hard work was done in a superb first half when the Hornets, who were almost rampant at times, scored through on-loan duo Heidar Helguson and Tom Cleverley.
The outcome might have been different had Jon Parkin's shot in first-half injury time gone in, rather than wide, off a post. But after the break the home side showed a high degree of defensive discipline to such a level that keeper Scott Loach only had to make one save of note.
Watford started well and deservedly took a ninth-minute lead through Helguson. Making the first start of his second spell at Vicarage Road, the Queens Park Rangers forward pulled away at the far post to head a Don Cowie cross back across keeper Andy Lonergan and into the far corner.
Watford continued to create chances and a second goal was coming, but when it finally arrived in the 33rd minute it was well worth the wait.
Henri Lansbury picked the ball up in midfield and attacked the heart of the visiting defence before finding Helguson, who turn and slid the ball to his left where Manchester United youngster Cleverley struck a low first-time right-footed effort inside Lonergan's near post.
But having been totally dominant, Watford nearly gifted their opponents a route back into the game in first-half injury time when Adrian Mariappa scuffed a clearance to Billy Jones, whose mis-hit effort fell for Parkin, and the big striker turned and hit a shot that beat Loach but bounced wide off his left-hand post.
The visitors did improve after the break, although they could hardly have got much worse, but they had to wait until late on for a chance of note when Parkin forced Loach into a fine diving save.
Watford did have other opportunities - Helguson and Lansbury went close with a header and free kick respectively - but they were largely content with their lot and never really looked in danger of relinquishing it.