The Collaborative International Dictionary of English v.0.48:

Vassalage \Vas"sal*age\, n. [OE. vassalage, F. vasselage, LL.
   vassallaticum.]
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   1. The state of being a vassal, or feudatory.
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   2. Political servitude; dependence; subjection; slavery; as,
      the Greeks were held in vassalage by the Turks.
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   3. A territory held in vassalage. "The Countship of Foix,
      with six territorial vassalages." --Milman.
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   4. Vassals, collectively; vassalry. [R.] --Shak.
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   5. Valorous service, such as that performed by a vassal;
      valor; prowess; courage. [Obs.] --Chaucer.
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WordNet (r) 3.0 (2006):

vassalage
    n 1: the state of a serf [syn: serfdom, serfhood,
         vassalage]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "vassalage":
   absolutism, bond service, bondage, captivity, control,
   debt slavery, deprivation of freedom, disenfranchisement,
   disfranchisement, domination, enslavement, enthrallment, feudalism,
   feudality, helotism, helotry, indentureship, peonage, restraint,
   serfdom, serfhood, servility, servitude, slavery, subjection,
   subjugation, thrall, thralldom, tyranny, villenage