The Northern Ireland midfielderpounced at the far postin what was almost the lastaction of the game, noddinghome after substitute AndySlory had headed the ballacross to the far post.
It maintained Albion's recordof scoring in every game thisyear but means they still needtwo more wins to make sure oftheir fourth promotion to theBarclays Premier League in thepast eight years.
Late show: Chris Brunt celebrates scoring his last-gasp equaliser
At the other end of the table,Mackay was left cursing hisluck. Traditionally, the bounceof the ball does not favourteams embroiled in a relegationscrap.
So the Scot perhapsshould have seen it coming.Forced into an early re-shuffleof his defence, he then sawTom Cleverley carried off on astretcher following a clash withJonas Olsson.
If that wasn'tenough, Jon Harley was sentoff for a second yellow card 12minutes from time.But the 10 men rallied andDanny Graham's magnificent25-yard volley five minutes fromtime seemed to have lifted theclub clear of the drop zone.
Brunt had the final word,though. Two seasons ago, itwas the winger's goal thatsecured West Brom's third promotion.
Great leveller: Chris Brunt heads beyond Scott Loach to equalise for West Bromwich
It did not have thesame effect at Vicarage Road,but on the overall balance ofplay, it was deserved.
West Brom boss Roberto DiMatteo refused to get carriedaway, insisting: 'There's a lot ofexcitement around.
'We have put ourselves in agood position but we are notthere yet. We have to stayfocused and not listen toomuch to what's going onaround us.'
Di Matteo's men now head toDoncaster on Saturday in theirbid to clinch a return to thePremier League, while Watfordface a relegation six-pointer athome to rivals Plymouth.Watford remain just out ofthe relegation zone on goal difference.
But they should havebeen clear after Don Cowie'sknockdown was seized uponby Graham to fire home.
Hismanager insisted afterwardsthat the forward had notscored a better goal.
Row: John Eustace and Graham Dorrans square up
'You could say that it didn'tlook like being our day,' he said.
'But there are no hard-luckstories here. I'm immenselyproud of the players. You reallywould not have been able totell which team were going forpromotion from that game.'
Mackay did not reveal theseverity of Cleverley's injury buthe did confirm that the on-loanManchester United youngsterhad not been taken to hospital.
The Watford boss added:'There's walking wounded andice packs everywhere. They willbe assessed over the course ofthe next two days.'