We claim the ornamental design for a vertical grip attachable to a rail of a firearm, as shown and described.
FIG. 1 is a perspective view of a vertical grip attachable to a rail of a firearm;
FIG. 2 is a perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a side view thereof;
FIG. 6 is a side view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a top view thereof.
The surfaces outside the scalloped solid line perimeter of the front and back grip features do not form part of the claimed design; the broken lines show portions of the article that form no part of the claimed design.