We have almost 3,000 sales of this theme and no one else has had the problems you describe, so this suggests that the issue is unique to your setup.
Do you have additional plugins installed and activated? If so, try deactivating all plugins, all at the same time, and then see if you still have the problem.
Try downloading the full package because there is documentation in there that will help you to use the theme once you get it installed. When you reinstall the theme, use FTP to delete the ‘wp-content/themes/parallelus-vellum’ folder on your server, and then install the Vellum theme via the WordPress interface as this is a more reliable way to install themes than via FTP.
Remember, don’t use the Customize panel to save settings because that will result in many settings being reset to defaults. Please stay away from the Customize panel until we publish a theme update.