Visual bell is a metacity feature.

It does the same thing in other window managers. I don't see any interaction with the window manager here:

void
_vte_terminal_visible_beep(VteTerminal *terminal)
{
	GtkWidget *widget;

	widget = &terminal->widget;
	if (GTK_WIDGET_REALIZED(widget)) {
		/* Fill the screen with the default foreground color, and then
		 * repaint everything, to provide visual bell. */
		gdk_draw_rectangle(widget->window,
				   widget->style->fg_gc[GTK_WIDGET_STATE(widget)],
				   TRUE,
				   0, 0,
				   widget->allocation.width, widget->allocation.height);
		gdk_flush();
		/* Force the repaint. */
		_vte_invalidate_all(terminal); /* max delay of UPDATE_REPEAT_TIMEOUT */
	}
}

Hmm. I thought you were talking about the window manager feature that flashes the title bar instead of beeping.

vte_terminal_set_visible_bell() indeed doesn't do any real delay.

*** This bug has been marked as a duplicate of bug 602726 ***