Hi Frank,
If you are on Joomla 3.9 you should update to Visforms 3.12.7 and the subscription 3.3.8.
With that subscription version, setting up the custom mail is much easier. See:
www.vi-solutions.de/en/documentations/do...ding-mail-recipients
Now about your questions. The Mail To, Mail CC, Mail BCC, in the Form's Mail Options need not to be empty. If they are set, mails are always send to these addresses. Please make sure, that there is always a mail to available. (From the Mail To in the Form's Mail Options or through the plugin settings.)
If you stay with the older Visforms and subscription versions, you must manage custom redirects for all forms in one script!
If you have difficulties making this work, check your script against a json online validator to make sure the syntax is correct.
Regards Aicha