It actually depends on the Vitara - I've seen 99+ 1.6s (SQ416) with 15" rims, and I've been told of 2.0s (SQ420) also with 15" rims - my SQ420 has 16s and 15s will not fit.
If you have 16 rims now, look through the rim holes and gauge the clearance from the caliper to the rim - if it's less than 0.5", 15s will not clear - if you have 0.5 or more, try the rim and see.