If you can reinstall WordPress totally from scratch using a brand new database, that is basically the same test. But it must be a brand new database, we won’t learn anything if the new install of WordPress uses the existing database. Also, make sure you install Vellum using the WordPress interface as this will ensure that the correct server permissions are set.
I wish I had an instant solution for you, but no one has ever reported an issue like this before, and I can’t replicate the issue, so just at the moment I’m as clueless as you are. My support shift is ending now (it’s past 9pm here in the UK) so if we need to continue this discussion we will have to do that tomorrow.