Grepping my include directory, here's all the 'vsnprintf's that I found. Maybe include c.h? Draw your own conclusions, I guess; I've never used it.
kishna:/usr/include$ grep -i -n vsnprintf *.h c.h:802:#ifndef HAVE_VSNPRINTF_DECL c.h:803:extern int vsnprintf(char *str, size_t count, const char *fmt, va_l ist args); config.h:345:/* Set to 1 if you have vsnprintf() in the C library */ config.h:346:#define HAVE_VSNPRINTF 1 config.h:348:/* Set to 1 if your standard system headers declare vsnprintf() */ config.h:349:#define HAVE_VSNPRINTF_DECL 1 glib.h:1528:gint g_vsnprintf (gchar *string, pg_config.h:444:/* Set to 1 if you have vsnprintf() in the C library */ pg_config.h:445:#define HAVE_VSNPRINTF 1 pg_config.h:447:/* Set to 1 if your standard system headers declare vsnprintf() */ pg_config.h:448:#define HAVE_VSNPRINTF_DECL 1 pqexpbuffer.h:14: * It does rely on vsnprintf(); if configure finds that libc do esn't provide pqexpbuffer.h:15: * a usable vsnprintf(), then a copy of our own implementation of it will stdio.h:297:extern int vsnprintf (char *__restrict __s, size_t __maxlen,
