If you don't need a local copy of your Vault (due to storage limitations, or files that you don't need to access often, etc.) you can follow some steps to create a Vault and only have it online at files.billtobox.be, without a local copy of the Vault.
WebDAV
Billtobox Files also allows you to connect to your Vaults via WebDAV.
That way you don't need a local image file to access your data. For more information on how to set up a WebDAV connection to your Vaults, please have a look at the specific WebDAV articles.
files.billtobox.be
Create a new Vault (remember that you can create as many Vaults as you need for free)
Put the data in the Vault that you want to store online
When the Dashboard shows "in sync", all the data in the local Vault is also on files.billtobox.be
Next, remove the local copy of the Vault:
Open your Billtobox Files Dashboard
Click on the icon next to your Vault
Select remove
This will remove only the local copy, not the Vault on files.billtobox.be. If you need access to the Vault later on, you can re-add it to your Dashboard or access the data directly via the web interface at files.billtobox.be.