Vassals and Military Assistance

If a country is set as a vassal at the beginning of a campaign, how strong is it possible to make this alliance? Can the dominant power (player controlled) exercise his will effectively over said vassal, to demand troops, money etc? Will this all be carried out to the letter by said vassal?

A second question is, if the vassal state were player controlled, would it then be able to break the vassal relationship (asking as i have never really work with vassalage in any TW game). Would the non-player dominant state still supply demands on the player controlled vassal?

Thanks in advance.
Crassus