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 manager.
"You hope you get those decisions. I don't think anybody could believe he wasn't pulled back."
Lambert wondered whether Villa would have got the spot-kick had Helenius opted to go down rather than stay on his feet and go for goal, although he did recall a similar incident at Arsenal on the opening day of the season.
"I thought that myself," he said.
"But does it matter whether he has shot or not? At Arsenal the referee waited.
"I have certainly not seen anything like that before."
Even Villas-Boas conceded Moss had got it wrong.
"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."