Vanek was offered to the Leafs on Friday or Saturday, though it's unclear what the Sabres wanted in return, in addition to a first-round draft pick.
With the Leafs up against the salary cap, they would have had to move a couple of players to accommodate Vanek's cap hit of more than $7.14 million.
It's unclear if the two teams are continuing trade talks for other players, though. Leafs assistant GM Claude Loiselle and pro scout Tom Watt took in Monday night's Sabres 4-3 loss to the Dallas Stars in Buffalo.
The Sabres spoke with other teams about Vanek, but only the Islanders were willing to pay the price.
Vanek was traded to the Islanders on Sunday in exchange for left wing Matt Moulson, a 2014 first-round draft pick and a 2015 second-round pick. The Sabres are also paying 19.5 per cent of Vanek's remaining salary.
The Leafs, meanwhile, have held trade calls with other teams, and are actively looking to move a defenseman, presumably John-Michael Liles, though teams have inquired about Carl Gunnarsson, Cody Franson and Jake Gardiner.