Atticus is unable to directly import Vellum source files ending with the .vellum extension. However, you can export your Vellum file as a .docx document. This format is highly compatible with Atticus and will help you maintain a consistent and trouble-free formatting experience.
Once you've exported the .docx file, I’d recommend following this tutorial with our best recommendations for preparing your docx file prior to upload to ensure everything imports as planned: https://www.atticus.io/how-to-prepare-your-word-document-for-upload/