Chadli and Vertonghen are Belgium teammates, while he played with Dembele at FC Twente.
"They said a lot of good things about Spurs. They said 'you have to come'. I was very happy I made the transfer. I've already played with them for the national team and we have good contact. I expect a lot from myself this year and I hope I can make everyone happy," he told the Spurs website.