It is currently not possible (as far as I can tell) to assign a password to folder(s) from the Chrome extension interface once the password has already been created.

Our users interact with Vault almost exclusively within the extension, especially when clicking 'edit' on the password launches a large window of the Chrome extension rather than the Vault application itself.

It may also be worth considering having the extension launch the full application when a user tries to edit a password. Right now, it launches a new window, and I think users assume it's the full app. I know I did.