I thought I had forgotten my password and couldn’t find the recovery key, but in the end I managed to guess it. As the password is saved in the app for my desktop vault, I only want protection when accessing from eg a USB, is there any way of recovering it from the desktop app once it has mounted the vault? I suspect not 
no, there is no way.
The password is only used to decrypt the vault masterkey, which is responsible for en- and decrypting your data. Once the masterkey is decrypted (aka the vault is unlocked), the entered password is erased from memory.