Sandoval directed a header into the net off a Lovel Palmer cross just 9 minutes into the contest, and Salt Lake went on to record the win despite resting its biggest stars in preparation for Tuesday's US Open Cup final.
RSL (15-10-6) surged back into first place in the Western Conference with the victory.
Vancouver, meanwhile, fell to 11-11-8 and remained out of the playoff picture in the West, remaining four points out of the fifth and final post-season berth.
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