The Burnley striker, on as a 79th-minute substitute, struck three minutes into stoppage time as Sean Dyche enjoyed a happy return to Vicarage Road.
Dyche insisted there were no hard feelings despite being replaced by Gianfranco Zola as Hornets boss in the summer but he did his best to derail his old club's bid to reach the Premier League.
In fact, the warm reception he received from the home fans had barely died down when his Burnley side took the lead after just 39 seconds.
Charlie Austin got that one and went on to make it 28 for the season from the penalty spot after Troy Deeney had equalised.
But Fernando Forestieri levelled again at the end of a crazy first half hour and then fired Watford ahead after the break.
However, Vokes had the final say with his dramatic leveller as Burnley snatched a point.
Alan Dunne grabbed a late winner as Millwall banished any lingering relegation fears with a 1-0 win over 10-man Leicester at the King Power Stadium.
The home side were rocked by captain Wes Morgan's early dismissal but held firm for 87 minutes before seeing their promotion hopes suffer another blow as their winless run extended to six matches.
Millwall were disappointingly conservative considering their numerical superiority but took the spoils when Irishman Dunne popped up three minutes from time to slot home Richard Chaplow's cut-back.
Bristol City's dire away form continues to be a severe hindrance in their battle to stay in the league as they were beaten 3-0 at Derby County.
Goals either side of half-time from Jeff Hendrick and Jamie Ward, who saw a third-minute penalty saved by Tom Heaton, and an injury-time strike from substitute Ben Davies leaves the Robins rooted to the bottom of the table.
Jordan Rhodes rescued a point for managerless Blackburn, who extended their winless run to nine matches with a 1-1 draw against Lancashire neighbours Blackpool.
On-loan MK Dons centre-back Gary MacKenzie gave Blackpool a 64th minute lead as Rovers once again failed to convince. They looked set to rue earlier missed chances by David Jones and Josh King but Scotland international striker Rhodes secured a share of the spoils with his 24th Championship goal of the season.