So you wanna start the heist normally and play it through till you get the keycard from the bank manager. While you run through the server room door, continue running into the wall in front of it for about 3 seconds untill you can see through the wall. After doing this you may notice the frames going down every 5 seconds or so. This is normal and is required to proceed.

So now that the frames drop every 5 seconds or so, you wanna pick up the drill and thermite if you aldready haven't and walk towards the on the second floor which is directly above the vault gate (DO NOT PLACE THE DRILL). So wait for the next frame drop and run into the wall, where you will start to fall. You have to take this oppurtunity to turn around and run into the vault.
After succesfully making it in the vault, run to the room in which you pour the thermite and the frames will rapidly drop, take that moment to run into the wall away from the vault gate, and there you have it, you're inside the vault

If you do not manage to make it into the vault, you will just keep falling, and falling, and falling...
At this point every frame will be visible and it will look something like this: