In his desperation to recover the lost ground, Vertonghen stumbled and as he was falling made a grab for Helenius.
The Belgian succeeded only in grabbing the striker's shorts, yanking them down to his knees just as he was about to shoot.
Helenius carried on anyway, blasting over from an acute angle.
To Lambert's disbelief, referee Jon Moss opted to give a goal-kick rather than a penalty that surely would have been accompanied by Vertonghen's exit.
"It is the first time I have seen something like that happen," said the Villa chief.
"You hope you get those decisions. I don't think anybody could believe he wasn't pulled back."
Tottenham manager Andre Villas-Boas, whilst understandably delighted with the win, admitted his side had got lucky with Vertonghen staying on.
"It was a difficult decision for the referee," said Villas-Boas. "I can understand why Paul would not be happy with that decision.
"Most likely in this situation there is no advantage to be gained and the penalty has to be whistled.
"Probably we got away with it."