The Collaborative International Dictionary of English v.0.48:

Vadium \Va"di*um\, n. [LL., from L. vas, vadis, bail.] (Law)
   Pledge; security; bail. See Mortgage.
   [1913 Webster]

   Vadium vivum [LL.] (Law), a living pledge, which exists
      where an estate is granted until a debt is paid out of its
      proceeds.
      [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

VADIUM, contracts. A pledge, or surety.