Just a short update on the CryFS situation I mentioned a few days ago.
I was contacted by Sebastian (the maintainer of CryFS) and he said he has been actively working on the solution to the upgrading problem.
He has already implemented quite a few things that will be useful for Plasma Vault, and I will make the CryFS backend default again in Plasma 5.13 (after the LTS release) if these updates get released and packaged by the most popular distributions.
The main changes so far include automatic filesystem upgrades and error reporting through error codes. This will allow Plasma Vault to know exactly why opening a vault has failed to be able to properly communicate it through the UI.
It is awesome how the free/libre open source community sometimes works. :)