The Hornets were quickest out of the starting blocks, earning themselves a corner within the first two minutes.
From this set piece, Marco Silvestri was able to get a palm to Almen Abdi's delivery, with Deeney's subsequent follow-up ball proving too heavy for Forestieri.
The away side's first genuine chance came on the quarter-hour mark, with Rodolph Austin sending a shot just wide from the edge of the area.
The deadlock was eventually broken in the 20th minute, as Forestieri was able to head past Silvestri after the goalkeeper saved his first shot.
However, Leeds equalised just after the half-hour mark as Gabriel Tamas sliced the ball into his own net following a cross from the right flank.
Drama ensued just before the hour, as Giuseppe Bellusci hacked Forestieri down in the box to gift Watford a penalty.
The former Catania player saw red for his challenge, while Deeney made no mistake from the spot to put the home side back into the lead.
Things would get even better for the Hornets midway through the second half, as Forestieri embarked on a fine solo run, evading challenge after challenge before sending the ball trickling into the Leeds net for his side's third goal.
Watford got their fourth in injury time, with Tozser striking a free-kick from 30 yards into the Leeds net, and things got even worse for the beleaguered visitors when Sam Byram was also sent off for an off-the-ball incident.