Van Dijk was shown a yellow card by referee Anthony Taylor after the 76th-minute foul on Onana in which the Netherlands centre-back missed the ball and caught the Everton midfielder on the shin with his studs.
VAR did not take further action after Taylor booked Van Dijk, but Lampard said the referee should have been told to review his decision by Video Assistant Referee Darren England, with television replays clearly showing the full extent of the challenge.
"I love Virgil van Dijk, as a player he is fantastic," Lampard said. "But sometimes you mistime tackles, it was up on Amadou's shin and his foot was on the ground.
"I'm surprised it hasn't gone to VAR and the ref [hasn't] gone to look at it and made the correct decision.
"It doesn't matter now, but for me it was a red and that changes the face of the last 20 minutes.
"There was some talk before this about bad tackles and the referee just has a job to do, and so does VAR, I just think they got that one wrong in my opinion."
Everton, meanwhile, confirmed that a supporter has been issued with a stadium ban after being identified throwing a plastic bottle following the VAR decision to disallow a Conor Coady goal for offside in the second half.
The object, a plastic bottle, narrowly missed Liverpool manager Jurgen Klopp.
"I didn't see at the time, but I've seen it now, because it's still at the crime scene," Klopp said. "I thought it was a glass bottle and that would have been really dangerous. It was not. It was plastic."