The ornamental design for a vessel, as shown and described.
FIG. 1 is a perspective view of a vessel showing our new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The vessel is shown with a symbolic break in its length. The appearance of any portion of the article between the break lines forms no part of the claimed design.