I claim the ornamental design for a vacuum, as shown and described.
FIG. 1 is a top front perspective view of a vacuum according to my new design;
FIG. 2 is a front view of the vacuum shown in FIG. 1;
FIG. 3 is a rear view of the vacuum shown in FIG. 1;
FIG. 4 is a right side view of the vacuum shown in FIG. 1;
FIG. 5 is a left side view of the vacuum shown in FIG. 1;
FIG. 6 is a top view of the vacuum shown in FIG. 1; and,
FIG. 7 is a bottom view of the vacuum shown in FIG. 1.
The ornamental design which is claimed is shown in solid lines in the drawings. The broken lines in the drawings are for illustrative purposes only and form no part of the claimed design.