The ornamental design for a vent-mount holding device, as shown and described.
FIG. 1 is a right, front perspective view of a vent-mount holding device according to the design;
FIG. 2 is a left, front perspective view of the vent-mount holding device of FIG. 1;
FIG. 3 is a left, front bottom perspective view of the vent-mount holding device of FIG. 1;
FIG. 4 is a right, front bottom perspective view of the vent-mount holding device of FIG. 1;
FIG. 5 is a left side view of the vent-mount holding device of FIG. 1;
FIG. 6 is a right side view of the vent-mount holding device of FIG. 1;
FIG. 7 is a front view of the vent-mount holding device of FIG. 1;
FIG. 8 is a rear view of the vent-mount holding device of FIG. 1;
FIG. 9 is a bottom view of the vent-mount holding device of FIG. 1; and,
FIG. 10 is a top view of the vent-mount holding device of FIG. 1.
The broken lines in FIGS. 8 and 10 represent assembly clip holes, concentric circular surfaces, and a notched circular opening and form no part of the claimed design.