When you get this error the first time utilizing VanillaSoft's Mass Emailing integration with Verticle Response it means that you already have an account with Vetical Response that is utilizing the same email address as your VanillaSoft account is.  In this case you must contact Verticle Response and ask them to convert your Verticle Response username to be used through your VanillaSoft portal.  The phone number is 866-683-7842.