The ornamental design for the vial holder, as shown and described.
FIG. 1 is a perspective view of the vial holder, showing our new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a right side elevation view thereof;
FIG. 5 is a left side elevation view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines in the figures define the bounds of the claim and form no part of the claimed design. The claimed subject matter extends to the boundary but does not include the boundary.