Vanek -- a former first-round Sabres draft pick who has 254 goals and 243 assists in 598 career games with Buffalo -- was sent to New York in exchange for Moulson, the Islanders' first-round pick in 2014 and a 2015 second-round pick, the team announced.
Vanek, 29, has notched four goals and five assists in 13 games for the Sabres this season. The Islanders acquired him after dropping to 4-4-3 on the season following a 5-2 loss to Philadelphia Saturday.
Moulson, also 29, has collected six goals and three assists in 11 games for the Islanders this season. He has amassed 124 goals and 109 assists over 333 career games with New York and the Los Angeles Kings.
Both are scheduled to become unrestricted free agents at the end of the season.