The ornamental design for a vest, as shown and described.
FIG. 1 is a left side front perspective view of a vest showing my new design;
FIG. 2 is a front view thereof; and,
FIG. 3 is a right side front perspective view thereof.
The broken line immediately adjacent to and surrounding the photographic area defines the bounds of the claimed design and forms no part thereof. The broken lines depicting the remainder of the vest show features that form no part of the claimed design.