The ornamental design for a convertible topcover and roll bar for a vehicle, as shown and described.
FIG. 1 is a left rear perspective view of a convertible top cover and roll bar for a vehicle according to our novel design, the right rear perspective view being mirror symmetrical;
FIG. 2 is a front view thereof;
FIG. 3 is a top view thereof; and,
FIG. 4 is a left side view thereof, the right side view being mirror symmetrical.
In the drawings, the broken lines depict portions of a convertible top cover and roll bar for a vehicle and form no part of the claimed design.