Milutin Osmajic has four goals in his past three games for Preston after his double at Watford
Milutin Osmajic's double earned Preston only their second away win of the season and denied Watford the chance to move into the Championship play-off places.
The Montenegro striker, who also scored twice a week earlier to knock Charlton Athletic out of the FA Cup, scored a goal in each half to emphasise North End's superiority, with the Hornets scoring a late consolation through Rocco Vata.
Preston manager Paul Heckingbottom was sent off at half-time by referee Tom Neild, but the victory at Vicarage Road means they have now only lost two of their past 14 league games.
Watford have now lost their past three home games after going unbeaten for their opening 11 on their home turf.
To compound their poor display, they lost both key attacker Kwadwo Baah and his replacement Festy Ebosele through injury.
Preston had not won at Vicarage Road there for nearly 20 years, and their 2-0 victory at Cardiff last month was their only triumph on their travels this season - only bottom club Plymouth had fewer away victories in the Championship.
Watford's previously imperious home form has wobbled, however, and their win at Derby on Saturday is now their only victory in their past six outings in the league.
Both sides looked lethargic in the opening stages and the sight of Baah pulling up as he shaped to cross, and then limping out of the contest, did not help the Hornets' mood.
Preston looked the brighter and deservedly went ahead in the 17th minute, with Brentford loanee Jayden Meghoma taking much of the credit.
He picked the pocket of defender James Abankwah on Watford's right and crossed to the near post for Osmajic to turn his shot past Jonathan Bond.
Watford were fortunate to go in at half-time only one goal down as Preston carved them open repeatedly, with Mads Frokjaer-Jensen and Ali McCann being denied by desperate blocks and McCann then forcing a fine parry from Bond before seeing his follow-up header cleared off the line by Francisco Sierralta.
Heckingbottom was then red-carded for foul and abusive language in the tunnel at half-time.
Another simple ball over the top unhinged the home side again, with only Bond's flying save denying Osmajic his second as he ran clear of the defence.
With Baah's replacement Ebosele also forced off with an injury problem, Watford looked bedraggled and they paid the price on 56th minute as they tried to force the pace.
An attack broke down and another easy ball over the top, this time from Frokjaer-Jensen, sent Osmajic streaking clear again.
This time he gave Bond no chance with a stunning drive between the Watford keeper and his near post.
That goal, and a couple of substitutions, roused Tom Cleverley's team and in the last minute of normal time Vata skipped past a couple of challenges and found the far corner from 20 yards for a fine goal.
Post-match reaction
Watford manager Tom Cleverley told BBC Three Counties Radio:
"I keep banging on about those basics but we were just absolutely nowhere near it in the first half. They ran harder, they ran faster, they tackled harder, won every first contact and every second contact - and that's even before I start on us technically as a team.
"We are throwing games into the hands of the gods, always giving ourselves mountains to climb. It's what we are at the minute, a very inconsistent football team.
"I told the players before the game we have a great record when we go ahead so that should be extra incentive but we just don't look like a team that's willing to take that step at the minute."
On injuries: "I'm not sure of the extent of Festy (Ebosele) and Imran (Louza) but Baah looks like a significant injury.
"We were all on the same page that we needed to improve the squad before this game but I can say that with a bit more conviction now."
Preston assistant manager Stuart McCall told BBC Radio Lancashire:
"We trained this morning and discussed how we wanted to come and play and overall it was another strong defensive performance.
"There was a little slip - you don't let Vata come inside on his right foot, and it was a really good goal by the kid.
"But we're delighted with the victory but obviously with the level of performance as well."
