Ideas on vsprintf functionality?

Hi everyone,
For an email-alerting application I am finding that I need to have functionality similar to Server Side Includes (SSI) except for an SMTP server application.
As an example, I want the user to be able to construct an email or SMS message during run-time (not compile time) that can contain various variables.

I could realize this functionality if DC had a vsprintf function of the form:

int vsprintf(char *buffer, char *fmt, char *args);
instead of
int sprintf(char *buffer, char *fmt, … );

But before I start “carving up” stdio to add this … Has anyone done this? or Does anyone have a better idea? or (best) Will Rabbit Semiconductor add a vsprintf function soon (hint, hint)?