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 showing my new design;
FIG. 2 is a left perspective view thereof;
FIG. 3 is a left side view thereof; and,
FIG. 4 is a top view thereof.
The broken line(s) which define the bounds of the claimed design form no part thereof. Outside the broken line boundary, the de-emphasized and halftone showing of a motor vehicle illustrates environment and forms no part of the claimed design.