The ornamental design for a bumper for a vehicle, as shown and described.
1. Bumper for a vehicle
1.1 : Perspective
1.2 : Front
1.3 : Back
1.4 : Left
1.5 : Right
1.6 : Top
1.7 : Bottom
Reproduction No. 1.1 is a perspective view of a bumper for vehicles showing our new design; reproduction No. 1.2 is a front elevation view thereof; reproduction No. 1.3 is a rear elevation view thereof; reproduction No. 1.4 is a left side elevation view thereof; reproduction No. 1.5 is a right side elevation view thereof; reproduction No. 1.6 is a top plan view thereof; and reproduction No. 1.7 is a bottom plan view thereof.