The ornamental design for a vessel, as shown and described.
FIG. 1 is a perspective view of the vessel, showing our new design;
FIG. 2 is a front elevation view thereof, the rear elevation view being a mirror image;
FIG. 3 is the right side elevation view, the left side elevation view being a mirror image;
FIG. 4 is a top plan view thereof; and,
FIG. 5 is a bottom plan view thereof.
The vessel is shown with a symbolic break in its height. The appearance of any portion of the vessel between the break lines forms no part of the claimed design. The broken lines in FIGS. 1, 4 and 5 represent the portions of the vessel that form no part of the claimed design.