As pointed out earlier mirroring is a trivial task in Veritas. Unlike other volume managers we can mirror a volume of any size in only one line. A plex is a mirror, remember saying that? Just add a plex.
A little example. We've got a 50G striped volume containing only one plex (duh, it's unmirrored!). Our boss tells us she wants to get that 50G volume mirrored for safety. We tell her not to worry, you'll be done before she's back from lunch. You'd then add some disks (if they aren't there) to the system and initialize them into Veritas. Once you'd created your VM disks we'd want to build a striped plex, using vxmake or vxmirror, which ever you prefer. There is a special point to be made here: when adding a second plex to a volume, thereby creating a mirror, all of the plexes the same volume should be the same layout. Since we have a striped volume, we therefore must have a striped plex (because the plex handles the layout, not the volume) which means we need to create a striped plex from our VM disks. Once our striped plex exists we only need to associate it with the volume we want to mirror. The instant that you associate the plex you'll notice that vxprint reports the volume and plex as "SYNC", which means that the data from the first plex is pouring onto our new mirror. Once the sync finishes your fully mirrored! That easy. And if you can get the disks on the system without a reboot (like on Solaris) you could have done all this without a second of downtime!