Troy Deeney scored his 23rd of the season to give the visitors an early lead but Jeff Hendrick and former Watford defender Craig Forsyth put Derby ahead before Mathias Ranegie equalised minutes after Marco Cassetti was sent off.
But a great strike by Hendrick restored Derby's lead and Chris Martin netted his 23rd of the campaign as Derby equalled the 84 points set by Billy Davies' promotion-winning team eight years ago.
Watford went into the game knowing they had to win to have any chance of joining Derby in the play-offs and Deeney put them in front in the fifth minute with a clinical finish.
Derby's defence was opened up by an astute pass from Cristian Battocchio and Deeney advanced into the box to lift the ball over Lee Grant and into the roof of the net.
It provoked a strong response from Derby with Johnny Russell and Hendrick having shots charged down before Jamie Ward delivered a low cross from the right which Craig Bryson just failed to turn in at the back post.
Derby continued to push Watford back and after Martin had scuffed a shot narrowly wide, they were level when Daniel Pudin's attempted headed clearance fell to Hendrick who drove the ball past Jonathan Bond from 12 yards in the 28th minute.
The home side were swarming all over the Hornets and Ward should have put them ahead three minutes later when he broke through on the right but his shot cannoned away off the legs of Bond.
Watford flickered briefly on the break but it had been all Derby after Deeney's opener and the visitors had to give the home side far less room down the flanks in the second half.
The visitors started after the break as confidently as they began the match but they were forced to make a change in the 52nd minute when Joel Ekstrand appeared to fall awkwardly and was replaced by Tommie Hoban.
Deeney was almost in when a sliced clearance flashed across the six yard area but Derby went in front on the hour when Bryson set up Forsyth to smash a rising shot past Bond from 12 yards.
It should have been three a minute later when a dreadful mistake on the edge of the area let in Martin but Bond blocked his shot and Ward drove the rebound just wide of the far post.
Watford's task became even harder in the 68th minute when Cassetti was sent off for a second yellow card but two minutes later they were level when substitute Ikechi Anya crossed from the right and Ranegie rose at the back post to head past Grant.
Bond made a terrific save from Martin and Anya almost set up Deeney as both sides chased a winner which arrived in the 82nd minute with a goal fit to settle any game.
Simon Dawkins played the ball in from the left touchline to Hendrick who turned inside before unleashing an unstoppable 25-yard shot that arrowed past Bond. Martin ended any arguments three minutes later when he slid a low shot from a tight angle past Bond.