Where someone has attacked vaults by attacking surrounding concrete and block or even a torch, some local guys have doen this.
They created a half cylinder as tall as desired wide enough to extend past the edges of the vault (say 6"). They would flange the cylinder on the inside to attach it to the wall. They would have a hole in the cylinder where the vault
door was and use a piece of tubing like sonotube the same diameter as the vault
door to keep that pathway open. They would also embed som anchors or bolts thru the wall inside of where the cylinder is, If your vaults are back to back thru bolts or rod will work. I would guess they had rebar in the cylinder as well. I don't tink it needs to be fancy. They then filled the cylinder with concrete which formed around as was secured to the wall with the bolts / rods. This basicaly adds an extra 6" of concrete all around the vault and the stainless makes a nice appearancce.
The recess also make working with a torch difficult. If you are using a laye
lock system make sure you can access both locks when done.
Slope the very top so any water runs off.