As Michael said. Always backup the masterkey.
When creating a new vault I do the following:
1.Create the vault, drop 1 small text file in there to make sure its working, then lock it.
2.Wait for cloud software to fully sync. (Should only take seconds unless you have an existing queue)
3.Create a ZipFile of the initial vault structure (consisting of the initial folder, masterkey and D subfolder)
4.Move the zipfile to an offline backup (or another cloud provider)
If you are using a password manager, such as Keepass, you could even store this as a secure attachment .
Occasionally the vault format is updated on version upgrades. (you will be prompted to upgrade your vault) You would need to backup your new masterkey after doing so.
Even without a backup ,many cloud providers have recycle bins, or document versioning if you accidently delete or corrupt your masterkey,