You can extend or shrink mounted VxFS file systems using the fsadm command. The size to which a file system can be increased depends on the file system disk layout version. A file system using the Version 7 or later disk layout can be up to 256 terabytes in size. The size to which a Version 7 or later disk layout file system can be increased depends on the file system block size.

See the fsadm_vxfs(1M) and fdisk(8) manual pages.