Even more so, most people, even in the army, don't even know who their VM is. Thus, it seems almost awkward and questionable when all of a sudden a regular knight starts sending out orders to the army. People second guess it since they lack a title of VM in their sig. It's like, "uh, should I actually be following this...? I think I'll just wait for orders from further up..."
Vice Marshals do have signatures when talking in the army.
