Washington, D.C. – Tom Soehn's return to RFK Stadium was one to forget Saturday night.
Soehn coached United from 2007-09, but the Whitecaps manager could not produce a good effort from his expansion club, which dropped its MLS-high 12th game of the season.
Vancouver (3-12-9) remained winless on the road in 13 matches this season, and lost for the ninth time in those games.
D.C. goalkeeper Joe Willis made his MLS debut in place of suspended Bill Hamid and made five saves to post his first-ever shutout and also had an assist.
United made it relatively easy on their rookie goalkeeper as Pontius scored in first-half stoppage time off an assist from De Rosario and D.C. piled on three more goals after the break.
Najar, the MLS Rookie of the Year last season, added to the lead three minutes after halftime when he ran onto a pass from Willis, then scored to the bottom- left corner from a tight angle for his third goal of the season.
De Rosario hit the post in the 66th minute, then Pontius followed with another goal - his seventh of the season - in the 70th minute. Pontius drove a rebound off the underside of the crossbar and in with the goal empty.
Substitute King ended the nightmare for Soehn, taking a pass from De Rosario before scoring from the middle of the area to the bottom right in the 81st for his first goal of the season.
United visits the Chicago Fire on Thursday in its next match, while Vancouver will try to regroup for its visit to the Portland Timbers on Aug. 20.