The Belgian international is convinced only those who work closely with Dembele can gauge his quality.
"I don't think people know how good Mousa is," Vertonghen told Tottenham's official website.
"You can ask every single player in the team and I don't think you can describe how good Mousa is.
"You can only know if you play with him or against him, see him train for a couple of weeks, then you'll find out."
"He's a special player and I can't think of anyone else who is a bit like him. For me, he's up there with the best two or three midfielders in the league.
"I certainly prefer to play with him than against him," Vertonghen added.