I claim the ornamental design for a vehicle diagnostic device, as shown and described.
FIG. 1 is a front elevational view of a vehicle diagnostic device showing my new design;
FIG. 2 is a back elevational view thereof;
FIG. 3 is a left side elevational view thereof;
FIG. 4 is a right side elevational view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof; and,
FIG. 7 is a perspective view thereof.
The broken lines in the drawings are shown for the purpose of illustrating portions of the vehicle diagnostic device and form no part of the claimed design.