The ornamental design for an extensionarm for a vise, as shown and described.
FIG. 1 is a perspective view from above of an extension arm for a vise
showing a plug at one end for insertion into a vertical bore in the top of
a vise jaw and showing at the other end a transferable plug having a jaw
face at its top end;
FIG. 2 is a perspective view from beneath of the device shown in FIG. 1.