The defender is out of contract in June 2013 and the Amsterdam outfit may have to accept offers from interested clubs this summer to avoid losing him on a free transfer
The Belgium international is out of contract in June 2013 and claimed earlier this month that he could stay in Amsterdam until his deal expires, despite mooted interest from the likes of Premier League trio Manchester United, Arsenal and Newcastle.
De Boer has stated there is still a chance that the defender could commit to a new contract, but has conceded that the club may be forced to sell in order to avoid losing him for free - news which is likely to put the English sides on alert.
"We know that his contract expires in 2013," he told Eredivisie Live. "So it's either renew or relocate to a good foreign club, where Ajax would also benefit from a transfer fee.
"We already told Jan last year in Turkey that we are keen to hold on to him. We will open talks early in March to discuss a new deal."
Vertonghen, who took over as Ajax skipper at the beginning of the season following the departure of goalkeeper Maarten Stekelenburg to Roma, has been linked with a number of other top European clubs, including Tottenham and Inter.