I've written the following in an attempt to help explain the Vassal System in the CIV: Warlords expansion. It may not be the most comprehensive explanation possible, but I hope it helps answer some of the many recurring questions which pop up in threads throughout the forums. Please feel free to add to or whatever in posts following.
Good Hunting.
~Crighton
The Warlords expansion introduces a new diplomatic option called Vassal State. Vassal states are like an "asymmetric alliance" between two Civs. Basically, one civilization serves as the master in the relationship and collects tribute from the weaker, vassal nation. After the discovery of Feudalism the Vassal State option becomes available in the Diplomacy menu.
