My machine has only one disk and it is not available as a backup target, why is this?

This is a good question.

The reason you cannot do this, is by design. Basically it is the same reason as why you cannot make backups of Time Machine on your local disk.
A backup made to the same disk as where your Virtual Machine resides is not a good backup when there are problems. For example when your disk dies or when your machine has problems. This however is not the only reason.

There are actually a few more reasons for not allowing you to backup to the same disk.
First there’s the reason that it can considerably impact your performance. Making a backup to the same disk as where your Virtual Machine is running does slow it down. Depending on the underlying disk system the VM might become very slow.

Then there is an even more important reason.
If you make a backup to the same disk as where your VM lives, creating the backup might cause your host to run out of disk space.
This is especially bad when a VM is running and is a potential cause of corrupting your VM. Yes, there are safety checks built-in against this scenario, yet you can still do this if you try hard enough. As a result we have opted for the safe solution instead of adding yet more checks and not allow this by default.

A possible solution

This does not mean that it is impossible, you can still do this, but using a slightly different technique. If you really want to make backups to your local disk then you can do that as follows:
Create an additional partition of a fixed size via Disk Utility on your main disk.
The additional partition is allowed to be set as a Disk Target by Vimalin even while it has the same physical disk. It still does have the potential of slowing down your VM while the backup runs, but a partition of a fixed size does not have the potential of causing your VM to run out of disk space while it is running.