Nguyen set up a tally by Diego Fagundez in the 61st minute, getting the ball to Kelyn Rowe who sent it on to Fagundez for a tally from near the penalty spot.
He added an insurance tally 10 minutes later for the Revolution (2-3-2), who picked up their first home win of the season.
Zac MacMath made five saves for the Union (3-3-2).
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