I claim the ornamental design for a vehicle step stool, as shown and described.
FIG. 1 is a front view of the vehicle step stool.
FIG. 2 is a side view of the claimed design.
FIG. 3 is a top view of the claimed design.
FIG. 4 is a bottom view of the claimed design.
FIG. 5 is a top perspective view of the claimed design; and,
FIG. 6 is a bottom perspective view of the claimed design.
The symbolic break lines in the drawings convey that the appearance of any portion between the symbolic break lines forms no part of the claimed design.