The ornamental design for an m1vest, as shown and described.
FIG. 1 is a front view of an M1 vest showing my new design;
FIG. 2 is a back view of an M1 vest showing my new design;
FIG. 3 is a side view of an M1 vest showing my new design; and,
FIG. 4 is another side view of an M1 vest showing my new design.
The disclosed embodiment relates to the ornamental design of an M1 vest. The broken lines shown in the drawings depict portions of the M1 vest that forms no part of my claimed design.