8mm stainless bolts with nyloc nuts and washers - but do some good research first so that you don't drill into brake lines or the fuel tank for example.....
John
Assuming the vault has fixing holes in the bottom drill corresponding holes in van floor, then drill two metal straps with bolt holes at the same centres weld bolts into straps making a long 'U' bolt type fixing.
With a helper install said straps with bolts welded on, from the underside of the van, screw nuts on protruding threads inside the vault, weld nuts to bolts. This should make vault immovable, other than using angle grinder to cut off nuts.
Hope you can follow this.
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