ICloud Drive has a 15Gb file limit: My Apple Aperture vaults are bigger than this but should work as they are packages, not files. Right or wrong?

You may be able to upload a vault to iCloud Drive, but if you try to update the vaults while on iCloud Drive the vaults will probably become corrupted. I tested with small Aperture libraries , iPhoto Libraries, and a small Aperture vault. None of them could be opened more than once after moving them to iCloud Drive. The libraries were corrupted and needed repairing. And worst, many managed originals were reported as missing.

Aperture libraries and vaults need to be on locally connected volumes.

Use locally mounted Mac OS X Extended volumes for your Aperture library

Is there a daily upload limit? - is a vault not a package (like the libraries themselves)?

A vault is essentially an Aperture library. If you replace the extension ".apvault" by ".aplibrary" you can use it as an Aperture library.

iCloud Drive looks like a regular drive, but it does more: See iCloud Drive FAQ

Here's what you can do with iCloud Drive:

• Store and access all of your documents in one place from any of your devices
• Keep files and folders up to date across all your devices
• Create new files and folders from iCloud-enabled apps
• Work on the same file across multiple apps

iCloud Drive is perfect for documents, but the photo libraries are packages with items linked by database files. When iCloud Drive is updating changes across your devices, it does not update the links consistently across all devices. Aperture and iPhoto require the libraries to be on locally mounted volumes, without any write access by other applications.