I claim the ornamental design for the vegetable keeper, as shown and described.
FIG. 1 is a perspective view of the vegetable keeper;
FIG. 2 is a front view of the vegetable keeper;
FIG. 3 is a top view of the vegetable keeper;
FIG. 4 is a left side view of the vegetable keeper;
FIG. 5 is a right side view of the vegetable keeper;
FIG. 6 is a back view of the vegetable keeper;
FIG. 7 is a bottom view of the vegetable keeper; and,
FIG. 8 is a perspective view of the vegetable keeper, showing in phantom a possible environment in which a vertical divider is included. The divider shown in phantom forms no part of the claimed invention.