OK, I got this working by rebuilding my RAID card's "Virtual Drive" setup. This time I set up a 2.2GB virtual drive dedicated for DSS and another virtual drive for the rest. Then I reinstalled VSS and it saw all of the available LUNs. This time I didn't use cfdisk because it was not needed in this scenario--I just let the VSS installer create its own partition.

I guess I was getting LUNs and partitions mixed up.