Given the following GIR definition:

      <virtual-method name="opened">
        <return-value transfer-ownership="none">
          <type name="none" c:type="void"/>
        </return-value>
        <parameters>
          <parameter name="error" transfer-ownership="none">
            <type name="GLib.Error" c:type="GError*"/>
          </parameter>
        </parameters>
      </virtual-method>


....

      <glib:signal name="opened">
        <return-value transfer-ownership="none">
          <type name="none"/>
        </return-value>
        <parameters>
          <parameter name="object" transfer-ownership="none">
            <type name="GObject.Error"/>
          </parameter>
        </parameters>
      </glib:signal>


vapigen will generate this:

                [NoAccessorMethod]
                public virtual bool opened { get; }

and not mark it as a signal.