Neither goalkeeper was troubled in the opening minutes as both Forest and Watford struggled to enjoy any sustained period of possession.
It was the visitors who had the first clear opportunity of the game but Deeney failed to give his side the lead, shooting wide from inside the box.
However, Deeney's mistake didn't live long in the memory as the forward provided a cross just a couple of minutes later for Vydra to convert easily from close range.
The home side's efforts were undone shortly before the break when Vydra broke clear of the Forest defence to give Deeney a simple opportunity to double the lead.
Dexter Blackstock should have pulled a goal back for the hosts on the stroke of half-time but the forward struck the ball wide of the target from six yards out.