The ornamental design for a decorative rearbumper of a vehicle, as shown and described.
The single FIGURE is a front and left side perspective view of a decorative rear bumper of a vehicle showing my new design, the front and right side perspective view is a mirror image of single figure shown.
The rear view of the surface that faces the automobile when installed is omitted and is not part of the claimed design.
The automobile portions shown in broken lines are for illustrative purposes only and form no part of the claimed design.