"In my opinion Tottenham have signed a great striker," Van der Vaart told Spurs' club website.
"He's still young but he's a real striker. He loves to play and work for the team but he doesn't just score goals, he makes goals as well.
"If you work hard at Tottenham and in the Premier League, people will love you and if you score goals, you will be a legend. I hope he will do both".
Janssen finished as last season's Eredivisie top scorer, scoring 27 goals in 34 games in the Dutch top flight and only time will tell as to whether he can replicate this form in England.