We don't have synonyms for vasal yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are nasal, vassal and basal.
(n) :
(rare) Alternative spelling of vassal. [(historical, law) The grantee of a fief, a subordinate granted use of a superior's land and its income in exchange for vows of fidelity and homage and (typically) military service.]
(v) :
(rare) Alternative spelling of vassal. [(transitive) To treat as a vassal or to reduce to the position of a vassal; to subject to control; to enslave.]
(n) :
(rare) Alternative spelling of vassal. [(historical, law) The grantee of a fief, a subordinate granted use of a superior's land and its income in exchange for vows of fidelity and homage and (typically) military service.]
(v) :
(rare) Alternative spelling of vassal. [(transitive) To treat as a vassal or to reduce to the position of a vassal; to subject to control; to enslave.]