There are several ways.
You may be able to move the tape between lpars, if it's assigned as desired (not required), but it depends on the tape controller. Look for dlpar.
If the tape controller is the same as the internal disks and you're using these disks for the vio then you can't move it.
If it's an external tape unit connected to an additional scsi pci card, you can move the tape between lpars.
Take a look at DLPAR (Dynamic LPAR).
Another way is to share the tape unit.
You can share the tape unit thru the vio server to the rest of lpars.
In the vio server you must add a virtual scsi adapter (server) that lets any virtual client adapter connect to it.
In each lpar you must add a virtual scsi adapter (client) connected to the vio server virtual scsi adapter.
In the vio server you must map the tape device to the vhost corresponding to the virtual scsi adapter you've just created.
This way, the first lpar that does a cfgmgr (or a reboot) will have the tape device assigned.
If a lpar has the tape drive assigned to it, you must remove (rmdev) the virtual tape device from the lpar (and I think also the vscsi virtual scsi client device), before doing a cfgmgr in another lpar to assign the tape.
Well, this is how it's done for the cd device, but I think it's more or less the same for a tape.
And there are other ways.
In a client, what we did is to schedule in each lpar a mkdvd (same as mksysb but to DVD format) to disk, instead of writing it to a DVD.
After that you can move or backup the mkdvd images to another lpar, a backup solution, or even a ftp or nfs repository.
When you need to recover one of the lpars, you can move its last mkdvd to a normal PC, burn it to a DVD and reinstall the lpar from it.