D.C. United (9-11-11) suffered its third consecutive loss and saw its wild card playoff chances reduced with three games remaining.
The expansion Whitecaps, long since eliminated from the playoffs, posted their second straight victory. They improved to 6-16-10 with two games remaining in their first MLS season.
Tan, the only Chinese-born player to play in the league, put the Whitecaps ahead 2-0 in the 47th minute as he headed in Alain Rochat's cross.
Camilo scored in the first minute for the Whitecaps while Brandon McDonald replied in the 58th for United.