I had trouble with this in the beginning also.
Volume Manager has to do with managing the disks, raid, striping, plexes, etc....
Veritas Filesystem is just that. Anything that you need to do to manage the data on the disks. Such as mkfs, mount, online resizing and defragmenting. It also gives you control over I/O, caching, and allocation.
Remember:
VM is managing the disks.
Veritas Filesystem is managing the data on the disks.