Why can I not open a backup directly?

As you might notice if you look at the backups, then there is no .vmx file. This is by design and not to lock you out of your data.

Actually the vmx file is still there, but the file is renamed into having a .vml file extension. The reason for this is to prevent users from opening their backup directly using VMware Fusion. Besides that being a bad habit, once you do that Vimalin can no longer verify the data as it will be changed by at least a few bytes and signatures made of the files no longer match.

So even if you stop using Vimalin, you can always still recover your backups by renaming the .vml file into a .vmx file.