From the resulting corner Vancouver somehow managed not to fall behind as Ned Grabavoy, Luis Gil and Devon Sandoval all had efforts blocked on the line before the visitors cleared the danger after the third attempt.
Sebastian Fernandez also went close for the visitors just before the break as he latched onto Omar Salgado's pinpoint cross at the back post, but with time and space he fired his first-time shot wide.
In the second half, Salt Lake upped the ante as Plata lifted a great cross to the back post for the run of Kyle Beckerman, but the midfielder couldn't direct his header on target.
Vancouver took the lead in the 73rd minute when Nat Borchers brought down Mattocks in the box with a late challenge. The striker took the kick himself and placed the ball into the bottom corner.
However, the home side earned a point as Beckerman picked out Plata on the left and the forward drove into the box before drilling an effort past Ousted into the net.