Accessing a mounted vault using the open/save dialog is more difficult with our own software compared to GNOME software! In the GNOME/GTK open/save panel, a mounted Plasma Vault will show up in the devices section of its Places panel, which is nice because it provides an easy way to access the vault when it;s mounted. The KDE open/save dialog does not, so you need to remember what location it's mounted to and navigate there manually.
Fixed with https://cgit.kde.org/solid.git/commit/?id=116c805ba461d55acc62884ab37f1b4e8f3b0c9d