Moulson and Vanek are in the final year of their respective contracts and can become unrestricted free agents next summer.
According to New York Newsday, the Islanders had no intention of signing Moulson to a new contract, which made him much easier to trade.
Newsday beat reporter Arthur Staple reports the Moulson and his agents were disappointed the Islanders didn't start contract talks this past summer. Under terms of the Collective Bargaining Agreement, Moulson could have signed an extension as early as July 1.
Furthermore, if Moulson was looking for a deal worth more per season than the $5.5 million the Isles are paying John Tavares, it wouldn't have happened anyway.
Moulson, who turns 30 on Nov. 1, likely won't be in Buffalo for too long, as the Sabres -- who are in a rebuilding phase -- could ship him elsewhere later in the season and obtain draft picks and/or young assets for him.