Created attachment 16314 [details] [review]
patch

Looks basically fine ... I'm not sure about the name
and signature. Having an out int as the first parameter
is distinctly unusual and _len normally signifies
a char*/len pair is being passed in.

I can't think of a better name, but for the signature
perhaps

 gboolean g_strdup_vprintf_len (gchar **buffer, gint *len,
                                gchar *format, ...);

(g_memdup_vprintf() would be an accurate name, but not
a particularly obvious one.)

How about

gchar *g_strdup_vprintf_size (const gchar *format, 
                              gint        *size, 
                              va_list      args)

I think as long as the name starts with g_strdup, we should keep the 
gchar * return. Otherwise, we could simply go for 

gint g_vasprintf (gchar       **buffer,
                  const gchar  *format, 
                  va_list       args) 

Of the ideas, I like the idea of a g_vasprintf() with exactly 
the same arguments as the GNU functions best.

I've added g_vasprintf() now.