As Gaurav suggests, the answer depends on the config.
If this is a native RHEL volume, then to put it under VxVM control (and in a diskgroup) you first need to convert it so VxVM can manage it.
You can learn more about converting here:
http://sfdoccentral.symantec.com/sf/5.0MP3/linux/manpages/vxvm/man1m/vxvmconvert.html
If this is a VxVM volume, then what diskgroup is it in today? What device(s) currently comprise the volume? If the volume (and its disks/LUNs) are the only volume in an existing VxVM diskgroup, a simple way is to 'join' the diskgroup. You can learn more about that process here:
http://sfdoccentral.symantec.com/sf/5.0MP3/linux/manpages/vxvm/man1m/vxdg.html
(see the 'join' keyword, also the 'move' keyword)
Regards,
Scott