We have not tested the VM1 w/ CasaTunes, but according to Russound, the VM1 integration should be seamless.
According to Russound, after linking the VM1 and your CAV 6.6 using the RNET ports, the VM1 will respond to any source selection events (which is what CasaTunes uses to change sources on the CAV 6.6) for zones 1 through 8 (where zones 7 & 8 are actually zones 1 & 2 on your second CAV 6.6).