I played a lot with multi vaults the last days and while not yet everything is there it's indeed super useful. I have similar use case as you describe: I want some files to be in a cloud storage (nextcloud in my case) but still have them in a Dendron vault. What you can do is to create a new vault via

Vault Add

and then edit the

dendron.yml

file and use the full path to your folder in the fspath element. If you're on Windows you need to replace the \ by / in the path.