Van der Vaart stated earlier this week that he did not enjoy the defensive side of his role after being selected on the right flank for Sunday's North London derby victory over Arsenal.
He also admitted he was not happy about being substituted and criticised Redknapp for commenting on his most recent injury.
Redknapp recognises that Van der Vaart is most effective in a central role, but his preference of late has been to use Defoe as a second main striker alongside summer signing Emmanuel Adebayor.
With both forwards performing well and Scott Parker, Luka Modric and Gareth Bale also in good form, Redknapp admits it is a problem fitting Van der Vaart into the side, especially now he has expressed feelings of discontent.
"Yes, it does seem to be a problem. Rafa can't run back and chase the full-back. Against the better teams it is a problem," the Spurs manager said in The Sun.
"Rafa's best position is in the middle, playing just behind the front man.
"The only problem is that Jermain Defoe plays there too and Jermain is playing very, very well at the moment.
"So, I guess I have got a decision to make, haven't I? I suppose it's now between Rafa and Jermain over who plays in the team."
Redknapp added: "Rafa is a terrific footballer. He has great skill. But if you're in the team and asked to do a job, really, you should just do it."