If your VM is named "FooVM", then you can create a snapshot using:
VBoxManage snapshot FooVM take FooVM_Snapshot1
Via command line you can configure where to store the snapshots:
VBoxManage configvm FooVM --snapshotfolder /media/foostick/foofolder
However, take care that the UBS hard drive is available all the time.
You can automate the backups using cron jobs. And there is no need to poweroff the VMs.