The ornamental design for a decorative vehicle side step, as shown and described.
FIG. 1 is a perspective view taken with respect to the decorative vehicle side step seen as an indeterminate number of a series of repeating flame patterns of which one is seen in solid line view and in which other multiples are shown in phantom and form no part of the invention hereof;
FIG. 2 is a side plan view of a vehicle's right side step, a vehicles left side step being a mirror image thereof;
FIG. 3 is a top view of a vehicle's right side step, a vehicles left side step being a mirror image thereof; and,