The ornamental design for a vessel, as shown and described.
FIG. 1 is a left-hand side view of a vessel comprising a new, original and ornamental design, the right-hand side view being a mirror image thereof, the portions shown in broken lines forming no part of the design;
FIG. 2 is a front view of a vessel comprising the new, original and ornamental design, the portions shown in broken lines forming no part of the design;
FIG. 3 is a rear view of a vessel comprising the new, original and ornamental design, the portions shown in broken lines forming no part of the design;
FIG. 4 is a top view of a vessel comprising the new, original and ornamental design, the portions shown in broken lines forming no part of the design; and,
FIG. 5 is a bottom view of a vessel comprising the new, original and ornamental design, the portions shown in broken lines forming no part of the design.