Defoe was signed from Tottenham in February, penning a four-year deal to be Toronto's designated player for the next four MLS seasons.
However, the England international was linked with a return to the Premier League before the transfer window shut following Ryan Nelsen's sacking last month.
Talk of a move back to England has refused to subside, especially with Defoe spending almost a month in his homeland nursing a groin injury.
And Vanney, who confirmed Defoe was close to a returning to Toronto to continue his recovery, admits he is unsure of the 31-year-old's future beyond January, when the transfer window reopens.
"In terms of Jermain in January, I don't know," Vanney told reporters.
"Maybe he comes back and he finds out that he really likes what's going on and the direction and he changes his mind. I have no idea where his mind's at, to be honest.
"I haven't had a sit-down, long conversation with him about things."
Vanney's comments come after Toronto CEO Tim Leiweke said Defoe will not be with the club next season during a talk at Ryerson University last week.
"We will have some new DPs [designated players] we got to go get next year, I predict," Leiweke revealed.
"I personally don't think Defoe will come back. I think if he doesn't want to be here, you get rid of him."