Ok, I picked up some Vaughan Williams CD's... I haven't found the opening music yet, but I've listened to a little... there's a lot of music that would be PERFECT for the war machine sequence. I also found a piece that would make a good theme for the thief.
So, the opening music is not part of his symphonies 2, 3, 5, 6, 8, 9. I guess I'll try his first, fourth, and seventh next.