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 VIVUM, contracts. A species of security by which the borrower of a
sum of money, made over his estate to the lender, until he had received that
sum out of the issues and profits of the land; it was so called because
neither the money nor the lands were lost, and were not left in lead pledge,
but this was a living pledge, for the profits of the land were constantly
paying off the debt. Litt. sect. 206; 1 Pow. on Mort. 3; Termes de la Ley,
h.t.