As nouns the difference between serfs and vassal
is that
serfs is while
vassal is (historical) the grantee of a fief, feud, or fee; one who keeps land of a superior, and who vows fidelity and homage to him, normally a lord of a manor; a feudatory; a feudal tenant.
As an adjective vassal is
resembling a vassal; slavish; servile.
As a verb vassal is
to treat as a vassal or to reduce to the position of a vassal; to subject to control; to enslave.