I just received an email that a new version of Vellum (3.0) will be released November 29, and will require macOS Catalina.

I own version 2.x.x, and I find it to be a wonderfully easy program to work with - but it was very pricey, so prospective owners should explore the features before committing. (I think the one thing the DEMO will not do is compile one's work for final ebook/print book.)

The new version (with a lot of new features) will be free for current owners.