Please go to the my-account page and login the vendor. Form that page, not from a special login modal or form. Login form the my-account page directly. Then test.
This often happens when a vendor logs in using a special form or theme created login, which does nto log the vendor in via the correct https
Also, if you have https, try installing the plugin: Really Simple SSL and run through the initial process of securing all content on your site. https://wordpress.org/plugins/really-simple-ssl/
