@icronos
First, try deactivating all plugins except WooCommerce, WC Vendors and WC Vendors Pro. Login as a vendor using the my-account page. See if this resolved the issue.
If not, try installing the Really Simple SSL plugin. https://wordpress.org/plugins/really-simple-ssl/ run through the options to secure content on your site.
Then try again to see if the issue is resolved.
Let me know.
Plugins such as wpsmush and other image plugins can sometimes disrupt the image uploaders.. as well as permissions and having insecure content on your site.