The ornamental design for an F1vest, as shown and described.
FIG. 1 is a front view of an F1 vest showing my new design;
FIG. 2 is a back view of an F1 vest showing my new design;
FIG. 3 is a side view of an F1 vest showing my new design; and,
FIG. 4 is another side view of an F1 vest showing my new design.
The disclosed embodiment relates to the ornamental design of an F1 vest. The broken lines shown in the drawings depict portions of the F1 vest that form no part of my claimed design.