SUMMARY
Currently, Dolphin never shows thumbnails for Vault files, apparently due to security concerns. However, for some users (myself included), thumbnails and previews are extremely important for navigation and file management. An option to securely enable thumbnails on a per-Vault basis would be quite helpful to most Vault users, and the ones who find it too much of a security risk could simply keep it disabled.

I don't have much experience with KDE development myself, but I'd be willing to try to help with implementing this feature.