The 29-year-old Vanek, a two-time 40-goal scorer, had four goals and nine assists in 13 games this season for Buffalo. In nine seasons with Buffalo, he had 254 goals and 243 assists in 598 games.
Both players are in the last years of their contracts.
Vanek's agent, Steven Bartlett, told ESPNNewYork.com that no contract extension for Vanek is in place with the Islanders. He doesn't expect any such talks to take place soon, either.
"He'll concentrate on going down there and doing well," Bartlett said when reached by phone Sunday night. "Hopefully, it will be a breath of fresh air."
The Sabres agreed to pick up 19.5 percent of what's left of the $6.4 million Vanek is still owed in the final year of his contract, according to multiple reports.
Vanek, a former University of Minnesota star from Austria, was co-captain of a struggling Sabres squad that has stumbled to a league-worst 2-10-1 start. With Vanek in the last year of a seven-year, $50 million contract and the Sabres intent on rebuilding, there has been speculation that he might sign with the Minnesota Wild this summer.
Should that happen, Vanek will turn out to be a very high-priced rental for the Islanders and general manager Garth Snow, who sent two high picks back in a deal intended to shake things up on Long Island amid the team's middling 4-4-3 start.
In acquiring Vanek, Snow dealt one of the team's leading goalscorers and core players.
Moulson, 29, who had six goals and three assists in 11 games this season, resurrected his career with the Islanders after toiling in the minors for a few years while playing with the Kings' AHL team. Moulson, an almost inseparable linemate with good friend John Tavares, posted three 30-goal seasons with the Islanders.
"Matt will address our current needs and provide us with support, leadership, scoring," Sabres general manager Darcy Regier said. "With where our hockey club is, the first- and second-round picks are important."
Moulson said he was "surprised" about the trade, especially that it happened this early in the season.
"It's sad to leave people you care about and love behind," Moulson told ESPNNewYork.com when reached by phone Sunday night. "I think over the years I've shown my want to be a part of the organization and turning around the team into being successful. I guess, obviously, in the long run, they didn't feel the same."
In the last year of a three-year, $9.4 million deal, Moulson is due a significant pay raise. He will likely command north of $5 million per year should he test free agency in July.
Though Moulson told ESPNNewYork.com earlier this season he'd like to remain an Islander, no progress had been made on an extension for the gritty goalscorer prior to Sunday's trade.
Moulson's agent, Wade Arnott, told ESPNNewYork.com that the Islanders and Moulson's camp had no substantive contract talks before the trade was made.
"I guess this is what happens in pro sports," Moulson said. "I gotta move on. Obviously, I'll miss everyone, but this will be a new chapter."
Sabres veteran goalie Ryan Miller also is in the final year of his contract and is another candidate to be traded.
"Like a lot of players he was shocked," Regier said of Vanek. "He was very professional. I thanked him. He was as good as he could be.
"It was a deal that was worked on and off for a while. It was something we felt we had to do."
Regier said he hasn't had recent trade talks regarding other potential free agents.
"Ryan continues to play extremely well," he said. "I probably talk to a third of the league of my counterparts on a weekly basis. You end up talking about a lot of things, and I'd leave it at that."
Moulson, who has a 14-month-old daughter and a newborn son, was packing up his things Sunday night with plans to join his new Sabres team Monday. He said he isn't very familiar with the team's roster but is looking forward to a new opportunity.
"Obviously not the start they wanted," Moulson said. "Hopefully I can be the guy that helps them turn that around."
Information from The Associated Press was used in this report.