Keane scored 16 goals and set-up 11 others for the Galaxy, who lost to Salt Lake in the first round of the play-offs.
Di Vaio netted 20 goals, five of them game winners and no penalties or free-kicks, while Magee, who joined the Fire after being traded from the Galaxy on 25 May, plundered 21 goals.
Cahill, a 33-year-old from Sydney who played for English Premier League side Everton from 2004 through last year, scored 11 goals and helped the New York Red Bulls claim the best regular season record in MLS this year.
Portland goalkeeper Donovan Ricketts, a Jamaican international, made the Best 11 after 14 shutouts, 92 saves and 2,880 minutes played over 32 matches.
Among those missing from the top-11 line-up were Galaxy striker Landon Donovan, snapping his run of five appearances in a row on the squad, and Vancouver forward Camilo Sanvezzo, who led the MLS with 22 goals in the regular season.
New York striker Thierry Henry, a French star who made the elite unit the past two years, was also overlooked.