I have dbus interface like this:

[DBus (name = "org.kde.StatusNotifierItem")]
public interface SNItemIface : Object
{
        /*More properties */
	/* Tooltip */
	[DBus(signature = "sa(iiay)ss")]
	public abstract Variant tool_tip {owned get;}
        /* More properties
}

When generating C code, I see this:
static GVariant* sn_item_iface_dbus_proxy_get_tool_tip (SNItemIface* self) {
	GVariant *_inner_reply;
	GVariant* _result;
	_inner_reply = g_dbus_proxy_get_cached_property ((GDBusProxy *) self, "ToolTip");
	if (!_inner_reply) {
		GVariant *_arguments;
		GVariant *_reply;
		GVariantBuilder _arguments_builder;
		g_variant_builder_init (&_arguments_builder, G_VARIANT_TYPE_TUPLE);
		g_variant_builder_add_value (&_arguments_builder, g_variant_new_string ("org.kde.StatusNotifierItem"));
		g_variant_builder_add_value (&_arguments_builder, g_variant_new_string ("ToolTip"));
		_arguments = g_variant_builder_end (&_arguments_builder);
		_reply = g_dbus_proxy_call_sync ((GDBusProxy *) self, "org.freedesktop.DBus.Properties.Get", _arguments, G_DBUS_CALL_FLAGS_NONE, -1, NULL, NULL);
		if (!_reply) {
			return NULL;
		}
		g_variant_get (_reply, "(v)", &_inner_reply);
		g_variant_unref (_reply);
	_result = g_variant_get_variant (_inner_reply);
	g_variant_unref (_inner_reply);
	return _result;
}

But Vala must not use g_variant_get_variant to this properties, because it is not of "v" type.