Unfortunately, there is not much we can do here.
The other plugin seems to aggressively output something in the response.
This is done even before we have a chance to intercept it apparently.
The only way to solve your issue would be to understand why the other plugin displays the message “your account is not setup as a vendor”.
Let us know how you manage to solve this.
You could try disabling the wcvendor plugin to check if that solves the issu to be sure,