The ornamental design for a capportion of a vial, as shown and described
FIG. 1 is a perspective view of a cap portion of a vial according to our new design;
FIG. 2 is a front view thereof, the rear view being a mirror image thereof;
FIG. 3 is a right side view thereof, the left side view being a mirror image thereof;
FIG. 4 is a top view thereof; and,
FIG. 5 is a bottom view thereof.
The broken lines immediately adjacent the shaded areas represent the bounds of the claimed design while all other broken lines are directed to environment and are for illustrative purposes only; the broken lines form no part of the claimed design.