When encapsulating the root disk with Volume Manager, this error message is printed.
This error message probably has nothing at all to do with the user's system PROM version. It most likely is related to the fact that the file /dev/vx/config (and the pseudo device that it is linked to) does not exist on the system. A few things could contribute to this file not being created:
Make sure these lines are in the /etc/system file:
forceload: drv/vxdmp (only needed for SEVM 2.5 and above) forceload: drv/vxio forceload: drv/vxspec
Make sure the vx entries are in the /etc/name_to_major file.
grep vx /etc/name_to_major
If you have not performed a boot -r since doing a pkgadd to the Volume Manager software, this might have contributed to the problem.