If I may refer to the JK Rolling’s Deathly Hallows book, we may get a better understanding of what happens in the vault. I say this because I am pretty sure JK Rolling wanted the movies to follow the books as close as possible.
In the book, the objects never stop multiplying. If you are touching something, even if it’s fake, it will multiply. The idea is that a thief will be buried alive if he tries to steal something. (In the book, an additional curse that creates heat would also roast you as you get buried.) The only thing that saves Harry, Hermione, Ron and the two goblins in the vault is its door is opened – by a swarm of armed goblins and wizards on the outside. The goblins plan on having their way with the thieves (maybe because they are unaware of the curses on the inside). So, the five are carried out of the vault in a wave of hot metal.
How do you capture that on the silver screen? The answer to that may explain why the plot was modified slightly, if you didn’t want to have loads of extras, many of which needed to be little people. So, the simple modification was that if you stop moving, the treasure stops multiplying.