DiVaio netted his first goal in six weeks in the 43rd minute to break a scoreless tie, using a fake by teammate Felipe Martins to put a shot into the top corner.
D.C.'s Conor Doyle scored the equalizer in the 81st minute, but DiVaio struck again only 2 minutes later off a feed from Patrice Bernier, curling a shot past United goalkeeper Bill Hamid for the game-winner.
The Impact (11-7-5) improved to 7-1 in their last eight games with DiVaio in the lineup.
United fell to 3-17-4.
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