  A general formula is derived for the finite volume dependence of vacuum
expectation values analogous to Luscher's formula for the masses. The result
involves no integrals over kinematic quantities and depends only on the matrix
element between pions at zero momentum transfer thus presenting a new way to
calculate the latter, i.e. pion sigma terms.
  The full order $p^6$ correction to the vacuum condensate $<\bar q q>$ is
evaluated and compared with the result from the Luscher formula. Due to the
size of the $p^6$ result no conclusion about the accuracy of the Luscher
formula can be drawn.
