Watford began brightly with Ashley Young and Anthony McNamee combining well to cause Stoke problems but the Potters defence kept their opportunities to a minimum.
Dave Brammer tried his luck in the wet conditions after 18 minutes when he struck a 30-yard low drive off the greasy surface but his shot dragged wide of Foster's goal.
Watford took the lead in style when Anthony McNamee's cross from the left picked out an unmarked Paul Devlin who confidently headed into Steve Simonsen's bottom left corner after 24th minutes.
Only two minutes later, a failed Brammer free-kick caused pandemonium in the box after Michael Duberry pumped the ball back into the danger area.
Lewis Buxton finally managed to strike a shot but unfortunately for his side, the ball cannoned back off of the crossbar.
City continued to push for the equaliser but still failed to hit the target before the break with both Martin Kolar and Junior going extremely close.
The visitors doubled their lead in the 67th minute when after Michael Duberry fouled Young, the forward stepped up and curled a superb free-kick into Simonsen's bottom left corner from 25 yards.
Young was the centre of attention again two minutes later when his run at goal was ended abruptly by the challenge of Simonsen, gifting the away side a penalty and the goalkeeper a red card.
Despite the sacrifice of Kolar for Ed De Goey, the Dutchman could only grasp at the ball as it flew into his bottom right corner from the boot of Marlon King on 72 minutes.
With the stuffing firmly knocked out of Johan Boskamp's men, Stoke could do little but slump to an embarrassing home defeat.