The ornamental design for a vacuum, as shown and described.
FIG. 1 is a bottom perspective view of a vacuum showing my new design;
FIG. 2 is a front and left side perspective view thereof;
FIG. 3 is a rear and right side perspective view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a rear elevation view thereof;
FIG. 7 is a front elevation view thereof;
FIG. 8 is a top plan view thereof; and,
FIG. 9 is a bottom plan view thereof.
The broken lines in the FIG. 9 represent the boundary between claimed and unclaimed areas. The white areas inside the broken lines form no part of the claimed design.