The ornamental design for a vacuum casing, as shown and described.
FIG. 1 is a perspective view of a vacuum casing showing my new design in a closed position;
FIG. 2 is a perspective view of a vacuum casing showing my new design in an open position;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a left side elevational view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The broken line showing of a handle and mounting plate, alongside multiple motors, fans, and apertures thereof is included for the purpose of illustrating environmental structure and forms no part of the claimed design. The broken line showing of an interior is included for the purpose of illustrating an arbitrary wall thickness of the vacuum casing.