What happens when your UpdraftVault quota fills up
If your quota allowance is full, no more backups can be sent to UpdraftVault until either some space is made (e.g. by deleting some backups) or more quota is purchased.
In the meanwhile, the backups will be kept on your web server (and fully usable).
Your UpdraftPlus backup report will inform you of this situation; make sure that you configure a backup report in your settings.