FIG. 1 is a top perspective view of a vial adapter having side windows in accordance with my new design;

FIG. 2 is a bottom perspective view thereof;

FIG. 3 is a right elevation view thereof;

FIG. 4 is a top plan view thereof;

FIG. 5 is a front elevation view thereof;

FIG. 6 is a rear elevation view thereof;

FIG. 7 is a left elevation view thereof; and,

FIG. 8 is a bottom plan view thereof.

The broken lines in the figures are for illustrative purposes only and form no part of the claimed design.