The Eredivisie side are refusing to give up on their efforts to keep both men on their books, but have accepted that the pair are eager to explore their options when the transfer window re-opens.
Promising centre-half De Vrij has been strongly linked with a move to Liverpool, with the Reds looking at the 22-year-old as a possible addition for the present and future.
Janmaat has also drawn interest from the Premier League, with north London rivals Arsenal and Tottenham Hotspur reported to have checked on the 24-year-old left-back.
Both men are expected to make the Holland squad for this summer's World Cup, offering them a global stage on which to further showcase their talents.
Feyenoord acknowledge that sales may then have to be sanctioned, allowing them to bring in important funds and clear a path for potential stars of the future to step out of their academy ranks and into the first team set-up.
Technical director Martin va Geel told Algemeen Dagblad on De Vrij and Janmaat: "Talks have stopped and we will wait for the summer.
"With De Vrij, we have already made the decision to wait until after the World Cup (before re-evaluating the situation)."