And filled with optical fibre by C&W / Mercury
Which neatly leads me to the idea of wrapping the entire vault with multiple passes of fibre optic (something like pigtail quality, maybe a bit stronger but definitely not armoured) Space this say every 6 inches but in a woven / grid pattern maybe with multiple passes with a few redundant fibres. Maybe cable tie it to the rebar. The vault door being the only area not protected. Light is fired off down the firbres to ensure their integrity and the concrete is poured. Any attempted breach of the walls stops the light being received and triggers the alarm.