It might also happen during the second when vault/core/model/version/vw.php is being uploaded. The file could be incomplete because it is still being uploaded at the time, and the class would not be found. There is nothing we can do about that. If it only happened once, and the time matches the time you were uploading, then it was probably something like that.
If anything, I think the only thing we can do about this is change the upgrade instructions to:
- Disable VaultWiki in your add-ons list.
- Upload the files.
- When the upload is finished, Re-enable VaultWiki in your add-ons list.
This is probably a good idea, since there could be a minute or two when the wiki will run with mixed file versions (which has been known to cause weird issues), if vault/core/model/vw.php is not the first file uploaded. Alternatively, the instructions can say to upload that file first, but the disable, re-enable instructions are probably easier for someone.