Andres Romero also scored for the Impact, who are 2-0-0 against the Salvadorian side after beating FAS in Montreal 1-0 earlier this month. Di Vaio scored the lone goal of that match.
Jorge Moran and Marcio Teruel replied for FAS.
Di Vaio's second goal of the night, on a penalty kick at the 60-minute mark, broke a 2-2 tie.
Romero opened the scoring, sending a pass from Di Vaio past El Salvador goalkeeper Adolfo Menendez seven minutes into the match. Di Vaio gave the Impact a 2-0 advantage one minute later on a pass from Ignacio Piatti.
But FAS stormed back, cutting Montreal's lead to 2-1 before halftime with a goal from Moran in the 33rd minute. Teruel scored on a penalty kick in the 51st minute to tie the game.
Montreal hosts the New York Red Bulls on Sept. 17 in its next CONCACAF match.
The Red Bulls open their CONCACAF play against FAS next week.