Wayne Rooney made an impressive start to life in MLS , while the LA Galaxy left it late to claim a win on Saturday.
D.C. was already ahead when Rooney came on, Yamil Asad having struck with a brilliant curling effort from 25 yards at Audi Field, where United was playing for the first time.
Rooney came on in the 58th minute and was involved in a fine team move that led to Paul Arriola doubling the lead.
Arriola struck again from 25 yards after receiving a pass from Rooney as Ben Olsen's side claimed just its third win of the MLS season, despite Alphonso Davies' late consolation goal for the Whitecaps.
Even with Zlatan Ibrahimovic rested, the Galaxy overcame the New England Revolution 3-2 at Gillette Stadium.
New England was reduced to 10 men after 23 minutes, Cristian Penilla shown a red card for kicking out at Romain Alessandrini after a foul near midfield.
Yet the Revs led twice thanks to Diego Fagundez and Luis Caicedo and looked on track for victory when Ashley Cole was sent off for the Galaxy with five minutes remaining.
However, there was late drama as David Romney headed in a 92nd-minute corner from Alessandrini, before Chris Pontius secured the Galaxy's win by completing his brace.
A quick-fire brace from Marc Rzatkowski helped the New York Red Bulls to a 3-2 win over Sporting Kansas City. Bradley Wright-Phillips had the Red Bulls out in front early on, but Johnny Russell had SKC level inside the opening 10 minutes. Roger Espinoza put Sporting out in front early in the second half, but Rzatkowski bagged two goals in a seven-minute span to earn the win for the home side.
Darwin Quintero was once again the man of the moment for Minnesota United as the club edged Real Salt Lake 3-2. The Colombian had a goal and two assists to put his side 3-0 up before Joao Plata struck twice late in the match to set up a bit of a nervous finish for United.
Toronto FC's rocky title defense continued with a 2-1 loss to Orlando City. It was a first win in 10 league matches for the Lions, who got goals from Chris Schuler and Dominic Dwyer to secure the victory.
The Colorado Rapids and Houston Dynamo played out a 0-0 draw, while Western Conference leader Dallas recorded a 3-1 win over the Chicago Fire and New York City continued its fine form by beating the Columbus Crew 2-0.