Lower, Mark A (1860) Patronymica Britannica: a dictionary of the family names of the United Kingdom. London: J.R. Smith. Public Domain.
VASSALL
As described in historical dictionaries
Patronymica Britannica (1860)
VASSALL. In the feudal system, a Vassal was one who held under another; more generally, one who was subject to an under-tenant or mesne lord. Hence the word became, in time, almost synonymous Avith slave.