The ornamental design for a taillight for a vehicle, as shown and described.
FIG. 1 is a front view of a taillight for a vehicle according to the design;
FIG. 2 is a left side perspective view thereof;
FIG. 3 is a left side view thereof; and,
FIG. 4 is a top view thereof.
The broken lines in the drawings illustrate environmental subject matter only and form no part of the claimed design. The drawings and descriptions represent the left side embodiment. The claim is also for a mirror image right side embodiment that is not shown. Also enclosed is a set of photographs of an embodiment of the design.