What about using VeeamZIP in order to achieve your goal?
VeeamZIP is similar to full VM backup. Nevertheless, when you perform backup with VeeamZIP, there is no need to configure a backup job and schedule it; instead, the backup process for selected VMs can be started immediately. Minimum settings are required during this type of backup: backup destination, compression level and enabling or disabling application-aware image processing.
All you need to do is to add your Linux NFS as backup repository to VB&R console and veeamzip given VMs to it. Afterwards corresponding .vbk file can be copied to external USB drive. In case of disaster, these VMs can be easily restored.
For more information regarding it see the following link:
http://technodrone.blogspot.com/2012/06/something-awesome-from-veeam.html
The aforesaid functionality is available in Free Edition.
Hope this helps.
Thanks.