Vassals and retainers are actually interchangeable words in English language Japanese history. If you search vassal or retainer in the English-Japanese online dictionary, Denshi Jisho, you'll find every Japanese word for these positions lists both vassal and retainer as meanings.

Vassal and retainer are both used in European medieval historiography. The usual distinction is that a vassal is a landowner who owes military service to a larger landowner, while a retainer is a personal follower of a lord, part of his household, and not necessarily a knight. Knights could be either or both. Many young knights were landless and essentially military servants of greater men, while some landed knights also served in the household of great lords or kings.