Rovers were made to pay the price for Herold Goulon's dismissal.
Sordell grabbed his first on the stroke of half-time, finishing off a swift counter-attack to bring an end to a low-key opening 45 minutes.
Billy Sharp deservedly headed Rovers level with his 10th goal of the season shortly after the break but Rovers' resurgence was firmly halted by the dismissal of Goulon for a second bookable offence just after the hour mark.
England Under-21 striker Sordell wasted little time taking advantage, glancing home a well-taken header to register his eighth of the campaign.
Substitute John Eustace added a late third with his first touch before on-loan Michael Kightly made it four in injury time to extend Watford's unbeaten run to eight matches.
Had the Hornets been able to register more than just three wins during that sequence they would find themselves higher than the lower reaches of the npower Championship table, while defeat for Doncaster - a fifth successive reverse on the road - sends the South Yorkshire club into 2012 facing a survival scrap.
The Hornets were marginally the sharper amid a scrappy opening quarter which saw both sides gifting possession to the other far too easily.
Sean Dyche's side came the closest to breaking the deadlock after 15 minutes, Rovers goalkeeper Gary Woods doing well to get down at his near post and turn the ball behind for a corner from which Nyron Nosworthy saw his header clip the top of the crossbar.
James Coppinger's sliced effort from the edge of the area proved no problem for Scott Loach, who was otherwise a spectator in the Watford goal.
But the chance did seem to spark Rovers' play and Sharp saw a well-taken lobbed effort land on the roof of the net in the 42nd minute.
Yet no sooner had Doncaster gone close than Watford broke the deadlock, Sordell clinically finishing a fast counter-attack from 20 yards.
There was still time for Coppinger to draw the visitors level but he could only fire wide when well placed.
Doncaster started the second half firmly on the front foot and duly drew level within two minutes of the restart, skipper El-Hadji Diouf getting to the byline on the left and delivering a cross which Sharp raced to meet at the near post from six yards.
A very different second half was played at a much higher tempo, Kightly and Giles Barnes exchanging efforts off target at either end.
And the encounter swung firmly in the Hornets' favour shortly after the hour mark as Goulon was given a second yellow card for a foul on Kightly.
The hosts were now firmly in the driving seat and soon got their noses in front, Sordell meeting Mark Yeates' cross with a glancing header which left Woods with no chance.
Troy Deeney wasted a great headed chance to make the game safe but Eustace did just that within a minute of stepping off the bench, capitalising on Herita Ilunga's poor clearance to rifle home from the edge of the area.
And Kightly made it four in the 92nd minute when he rounded off a flowing move from 20 yards.