But if the vault could be detached from the heavy ATM machine (105kg or 156kg), there's possibly the thief could steal the banknotes along with the vault.
The word of the day is lead, or any other dense metal.
In my limited knowledge I have seen the safe that holds the bills in some things have hundreds of pounds of lead bars on the bottom. By themselves they don't weigh that much, but 2 layers of them make it just about impossible to take it out without opening it and taking the bars out 1st. Unless you look like Lou Ferrigno in his prime.
BTAMs, ATMs, vending machines, anything with cash is going to be a target. Operators of these machines just have to figure out the balance of security vs. making it more secure but having to deal with the obsticales that making it more secure creates.
-Dave