The ornamental design for a cap for a drinkingvessel, as shown and described.
FIG. 1 is a top, right side perspective view of a new, original and ornamental design for a cap for a drinking vessel, wherein an upper cap member is positioned on a lower cap member;
FIG. 2 is a front elevation of FIG. 1;
FIG. 3 is a rear elevation view of FIG. 1;
FIG. 4 is a left side elevation view of FIG. 1;
FIG. 5 is a right side elevation view of FIG. 1;
FIG. 6 is a top plan view of FIG. 1;
FIG. 7 is a bottom plan view of FIG. 1;
FIG. 8 is a top, right side perspective view of the cap for a drinking vessel, wherein the upper cap member is removed from the lower cap member; and,
FIG. 9 is a top, left side perspective view of FIG. 8.
The broken lines in the drawings depict portions of the cap for a drinking vessel that form no part of the claimed design.