Alli joined Spurs from MK Dons in January but spent the remainder of the 2014-15 campaign on loan with his boyhood club.
The 19-year-old has enjoyed a fine start to his first season with Tottenham, and was on the scoresheet in Monday's 3-1 victory over Aston Villa.
And Vertonghen conceded after the game that he was not aware of Alli before his move to north London earlier this year.
"He's [Alli] a very good player," the Belgian told Sky Sports. "To be honest, before he came I didn't really know him.
"I didn't know anything about the lower leagues but this guy has impressed everyone since the first day we met.
"He was there for a short while at the end of the season but I never saw him play, but since the first training he's been unbelievable and I think it shows the strength of the leagues - the Championship, League One and League Two.
"I think he has a great future and I hope he stays with us for a very long time. He's a great player, great guy and I think he deserved to win this [man-of-the-match] trophy."