A couple of options...
1) If the VPS provider allows you to mount your own ISO and boot from it, then you can setup the OS from scratch and partition it how you want. You would simply create 2 partitions of the disk, install the OS on the first partition and leave the second partition as a blank block device and use its path in Enhance when installing the Backup role.
2) If you want a quick 'hack', you could create a predefined in size .img file somewhere within your filesystem and mount this as a loop device which will show as a block device.
Option 1 preferred. Option 2, in theory, should have none/minimal disk performance impact but at the end of the day, you're adding a 'extra layer' in the filesystem.
