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 lines immediately adjacent to and surrounding the photographic areas define bounds of the claimed design and form no part thereof. The broken lines depicting the remainder of the vest show features that form no part of the claimed design.