The ornamental design for a grainbin vent cover, as shown and described.
FIG. 1 is a perspective view of a grain bin vent cover showing my new design in use;
FIG. 2 is a perspective view thereof showing the cover broken away to display the internal structure;
FIG. 3 is a left side elevation thereof, the right side being a mirror image;
FIG. 4 is a top plan view thereof,
FIG. 5 is a rear end view thereof,
FIG. 6 if a front end view thereof; and,
FIG. 7 is bottom plan view thereof.
The broken line showing the grain bin in FIG. 1 is included for the purpose of illustrating use and environment and forms no part of the claimed design.