I must say, it's pretty damned impressive what you guys have done with it, especially figuring it out so quickly. I'd like to think it's because I coded it with some high-level concepts in mind, but more likely, you're PHP gods.
The installer's a +++++. What specific problems is it causing?
BTW, header duplication and problems will occur within the vbms_mail_message class. The API function you posted is just a wrapper to provide safe arguments for the class and to easily say "just send the message" without all the extra code.