The ornamental design for a front of a vending machine, as shown and described.
A description of the figure or each of the figures of the drawings is as follows:
FIG. 1 is a front left perspective view of a front of a vending machine showing my new design; and,
FIG. 2 is a front elevational view thereof.
The broken line showing of the rear portion of the machine, the indicia, the photographs, and the dispensed camera in FIGS. 1 and 2 is for illustrative purposes only and forms no part of the claimed design.