The term vassal state commonly refers to any state that was subordinate to another in the pre-modern international system. The vassal in these cases was the ruler, rather than the state itself. Being a vassal most commonly implied providing military assistance to the dominant state when requested to do so; it sometimes implied paying tribute, but a state which did so is better described as a tributary state.

The terms vassal state and tributary state are best reserved for the pre-modern era when there was no clear concept of modern state sovereignty. Today, what could be seen as the modern form of a subordinate state akin to a vassal state, is the puppet state.