In a VxVM environment a diskgroup is a “container” that has different entities ( subdisk , plex , volume ).
When a dg is created an entire disk is referred for this operation.
So , when the free space must be checked , this operation requires a dg control.

method 1:
root@system10/> vxdg -g rootdg free
DISK DEVICE TAG OFFSET LENGTH FLAGS
disk01 c1t1d0s2 c1t1d0 63968238 7156053 -
rootdisk_2 c1t0d0s2 c1t0d0 63968237 7156053 -

method 2:
root@system10/> vxassist -g rootdg maxsize
Maximum volume size: 14311424 (6988Mb)

The second method shows directly free space in Mb