The following vala code:

  void test (string format, ...) {
    string msg = format.vprintf (va_list ());
  }

is translated to this C code:

  void test (const gchar* format, ...) {
	va_list _tmp0_;
	va_list _tmp1_;
	va_list _tmp2_;
	gchar* _tmp3_ = NULL;
	gchar* _tmp4_;
	gchar* msg;
	g_return_if_fail (format != NULL);
	va_start (_tmp0_, format);
	_tmp1_ = _tmp0_;
	_tmp2_ = _tmp1_;
	_tmp3_ = g_strdup_vprintf (format, _tmp2_);
	_tmp4_ = _tmp3_;
	va_end (_tmp2_);
	msg = _tmp4_;
	_g_free0 (msg);
  }

with when compiled produces these errors:

debug.c:163:9: error: incompatible types when assigning to type ‘va_list’ from type ‘struct __va_list_tag *’
debug.c:164:9: error: incompatible types when assigning to type ‘va_list’ from type ‘struct __va_list_tag *’

which are these lines: 

	_tmp1_ = _tmp0_;
	_tmp2_ = _tmp1_;

Sincerely Fabian