Okay.
I’ve been under the Lincoln memorial twice and never saw vault
I took the tour in the early 80's at night with my two sons. There was no mention of a vault then. I'm wondering if they may be talking about opening the cornerstone. It was originally laid on February 12, 1915. Robert Todd Lincoln attended the ceremony. Normally when laying the cornerstone of a building, they put things from that time period in with the cornerstone.