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]