Rookie Hornets boss Sean Dyche, 40, took the reins at Vicarage Road earlier in the summer following Malky Mackay's departure to Cardiff.
And he was on course for a maiden victory when Craig Forsyth opened the scoring on the stroke of half-time and Mark Yeates added a second with 20 minutes remaining.
Yet the Clarets roused themselves in the final 13 minutes as Charlie Austin pulled a goal back before fellow substitute Treacy - a summer signing from Preston - headed an 84th-minute equaliser.
Burnley boss Eddie Howe opted to start with Lee Grant in goal ahead of Brian Jensen and also handed starting roles to on-loan Manchester City rookie defenders Kieran Trippier and Ben Mee.
Treacy started on the bench, while Dyche named striker Chris Iwelumo in his side following his summer arrival from Burnley.
The imposing frontman completed the switch for an undisclosed fee and shook off a hamstring strain to start.
The Clarets, led by new skipper Chris McCann in the heart of midfield, made a bright and purposeful start.
They threatened in the fifth minute when midfielder Dean Marney strode forward and hit a fierce right-foot shot which sailed just wide from 20 yards.
In the 10th minute, Burnley winger Ross Wallace hit a 12-yard shot which drew a fine block from Adrian Mariappa.
The hosts were firmly in the ascendancy and moments layer Jay Rodriguez engineered the space to a hit a shot from six yards which missed the target.
In the 16th minute, the pendulum swung in Watford's favour when Iwelumo tried his luck from 18 yards with a shot that Mee did well to block.
Five minutes later, Burnley striker Martin Paterson tried his luck from 20 yards and Hornets goalkeeper Scott Loach made a comfortable stop.
Rodriguez then missed the target from 12 yards and Sordell also threatened from close range before Watford opened the scoring on the stroke of half-time.
Sordell engineered the space on the right flank to flight a cross which Forsyth headed into the bottom corner from six yards.
Burnley made a tactical switch at half-time as Paterson was replaced by Austin in a bid to boost the home attack.
Yet Watford continued to defend strongly as Austin wasted a decent opening before Forsyth shot wide when well placed.
Yeates came close to scoring in the 54th minute when he fired goalwards from six yards and drew a smart stop from Grant in the home goal.
His endeavour was rewarded in the 70th minute, however, when he drilled home a right-footed shot from 12 yards which gave Grant no chance.
But the Clarets responded in the 77th minute when Austin fired home from six yards and, with six minutes remaining, Treacy nodded in a cross from fellow wideman Wallace to level the scores.