Despite a bright start from the visitors, it was the hosts who broke the deadlock in the 14th minute when Laurent Ciman's speculative volley rebounded off the crossbar to give Jack McInerney an easy finish from close range.
Vancouver almost hit back when Kekuta Manneh found the space to curl a shot a few inches over the bar.
The Whitecaps piled the pressure on in the closing stages of the first half and they were left frustrated once again when Darren Mattocks rattled the post with a powerful strike.
Vancouver's shot-stopper came to the rescue once again when Ousted somehow managed to keep out Maxim Tissot's effort.
The saves proved to be crucial as Vancouver were gifted the chance to earn a point when Romero brought down Jordan Harvey inside the box.
Pedro Morales took on the responsibility from 12 yards and the substitute made no mistake as he beat Bush with the penalty to get his side back on level terms.
However, the lead lasted just four minutes as the Impact hit back to clinch all three points when Romero recovered from his error to spot the run of Piatti, who held his nerve to slot home the winner.