Troy Deeney's neat finish after 28 minutes, his 21st league goal of the season, and a stoppage- time strike by Matej Vydra gave Watford breathing space at the head of the battle for promotion to the Premier League.
Watford, who have one game left, are four points ahead of Bournemouth and Middlesbrough and six clear of Norwich City who all have two matches to play.
Boro play at Fulham later on Saturday and Norwich travel to Rotherham. Bournemouth, in second, host Bolton Wanderers on Monday.
The teams finishing first and second will be automatically promoted to the Premier League next season and the next four sides will go into playoffs for the last spot in the top flight.