Longtime Italian striker Di Vaio -- scoreless in his first eight MLS games since joining Montreal -- finally notched a goal in the 48th minute after Justin Mapp found him alone on the left side.
The Impact (8-13-3) doubled the advantage 2 minutes later on a Davy Arnaud goal off the second of three assists dished out by Felipe Martins.
Sanna Nyassi added an insurance tally in the 75th minute on an assist from Di Vaio as Montreal moved to within four points of a playoff position.
Thierry Henry netted his 11th goal of the season for Eastern Conference-leading New York.
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