With valac 0.11.2.76-50446 (git master, of this writing), the following Vala code generates the following invalid C code:

  [PrintfFormat ()]
  public static void print_line (string format, ...)
    {
      string output = format.vprintf (va_list ());
      stdout.printf ("%s%s\n", Utils.indentation_string, output);
    }

=>

void folks_inspect_utils_print_line (const gchar* format, ...) {
  va_list _tmp0_;
  /* XXX: these invalid duplicate variables aren't really used and cause a
   * compiler error */
  va_list _tmp1_;
  va_list _tmp2_;
  gchar* _tmp3_ = NULL;
  gchar* _tmp4_;
  gchar* output;
  g_return_if_fail (format != NULL);
  va_start (_tmp0_, format);
  _tmp1_ = _tmp0_;
  _tmp2_ = _tmp1_;
  _tmp3_ = g_strdup_vprintf (format, _tmp2_);
  output = (_tmp4_ = _tmp3_, va_end (_tmp2_), _tmp4_);
  fprintf (stdout, "%s%s\n", folks_inspect_utils_indentation_string, output);
  _g_free0 (output);
}

==>

==========================================
utils.c: In function 'folks_inspect_utils_print_line':
utils.c:375: error: incompatible types when assigning to type 'va_list' from type 'struct __va_list_tag *'
utils.c:376: error: incompatible types when assigning to type 'va_list' from type 'struct __va_list_tag *'
==========================================

Removing _tmp1_ and _tmp2_ and replacing instances of '_tmp2_' with '_tmp0_' in the C fixes the problem.