The ornamental design for a wall-mounted auxiliary vacuum unit, as shown and described.
FIG. 1 is a top front perspective view of a wall-mounted auxiliary vacuum unit showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a left side elevational view thereof;
FIG. 4 is a right side rear elevational view thereof;
FIG. 5 is a top plan view thereof; and,
FIG. 6 is a bottom plan view thereof;
The flexible hose and compartment door panel shown in broken lines in FIG. 1 are for illustrative purposes only and form no part of the claimed design.