Short answers, pulled from the story.
A vassal state is a state with a mutual obligation to a superior power, distinct from a colony because it maintains some degree of internal autonomy. Modern equivalents include the terms puppet state, protectorate, client state, associated state, and satellite state. Unlike a colony, a vassal state typically retains its own ruler and cultural identity while paying tribute or providing military service to the dominant empire.
The Amarna letters are a collection of 350 cuneiform tablets documenting correspondence between the Pharaoh and vassal rulers during the reigns of Amenhotep III and Tutankhamun, spanning 1390 BC to 1323 BC. They reveal the power dynamics of the relationship, including the grovelling tone vassal rulers used when addressing the Pharaoh. The letters are the primary source for understanding how Egypt administered its vassal states in the Levant.
Rib-Hadda was the ruler of Byblos and the most prolific letter-writer among ancient Near Eastern vassal rulers. His correspondence with Egypt spans twelve years, making it the longest documented exchange between Egypt and any vassal state. Despite his loyalty and the volume of his appeals, he never received a meaningful reply from Egypt and was eventually exiled from his kingdom by his own brother.
Hittite vassal treaties were imposed unilaterally by the Hittite king and required renewal whenever either ruler changed. Vassals owed military service and often annual tribute, while the Hittite king promised military protection in return. Treaties were frequently sealed by marriage between the vassal ruler and a Hittite royal princess, whose descendants would hold succession rights.
Arabia assisted Cambyses II in his invasion of Egypt in 525 BC and was rewarded by being exempted from formal satrap status. Instead of regular tribute, Arabia provided 1000 talents per year, as recorded in the Behistun inscription and the Persepolis Fortification Tablets. Herodotus records that during Xerxes' invasion of Greece, the Arabians served under Arsamenes, the son of Darius I.
The Qing dynasty classified Joseon as an autonomous vassal state, but the designation was contested. William W. Rockhill argued the label was misleading, describing Korea's tribute payments as fees for favourable trade access rather than marks of subordination. Rockhill also documented that Korea viewed the relationship in familial terms, likening the Ming dynasty to a father and the Qing dynasty to an older brother.