I claim, the ornamental design for an exterior surface configuration of a vehicle taillight, as shown and described.
FIG. 1 is a perspective view of an exterior surface configuration of a vehicle taillight showing my present design;
FIG. 2 is a front view thereof;
FIG. 3 is a right view thereof; and,
FIG. 4 is a top plan view thereof.
The broken lines shown in the figures represent portions of an exterior surface configuration of a vehicle taillight that form no part of the claimed design.