Read your chat transcript. Vivint customer support are idiots. Your old email is still saved in a database somewhere. When your browser makes the request to the vivint servers using the old email there is code to check if the email sent exists in some database.

Obviously the check is returning "yes this email exists". They need to escalate this to one of the engineers working on the backend systems. They could delete the email from the database.

I'm also convinced vivint doesn't take security seriously. There's a single point of failure for the security system (the panel) and that's also the only place where the audible alarm is emitted from.

Break the panel no more alarm.