Having a central defender as captain works because they can see the full picture - in Vinnie's case we have an articulate, intelligent, academic. intimidating (in the right way) and charismatic figure that no-one in world football can bully. The entire team looks up to him. A midfield captain can dictate proceedings and decide to adjust things if necessary. Strikers as captains - usually nothing more than ceremonial (like an Honorary degree) - e.g. Shearer.