I claim the ornamental design for a vest pack, as shown and described.
FIG. 1 is a side perspective view of a vest pack;
FIG. 2 is a front plan view of a vest pack;
FIG. 3 is a back plan view of a vest pack;
FIG. 4 is a side plan view of a vest pack;
FIG. 5 is a front view of a vest pack in use; and,
FIG. 6 is a back view of a vest pack in use.
The broken lines in the figure shown on the surface of the vest pack depict stitching and are included in the design claim; the broken line showing of a human figure in FIGS. 5 and 6 forms no part of the claimed design.