Heres an easy way to get this achievement aswell as the 'Lightning Looter' achievement aswell.
Now, once you start run down the stairs and into the hole in the wall, this will lead you to the room full of money. Kill the cops in here (Blindfiring works really well here) and loot as much money as you can. As soon as the enemy AI leave the hole in the wall you should head out, but if you look to the left before you leave the vault theres a pillar with a fire extinguisher, take the extinguisher. Once you leave the vault through the hole you will see many of the AI are huddled up together by the front of the train and some are up taking cover behind the pillars. What you do here is take cover by a pillar (preferably the further back one) throw the gas can into the enemies taking cover by the pillars and shoot it, this will clear all the enemies on the actual platform (hopefully, if not shoot the stragglers) now the remaining criminals will still be cowering in fear in front of the train, simply lean out of cover and blast them all to pieces. This shall hopefully kill them all. now loot the remaining money from their bodies and head up the stairs (the easiest is the right most stairs) once your on top of the stairs you will be in the lobby area. Blindfire the police around you and head to your right up the short stairs to the surface. Your now on the street, blindfire the cops and make a break for the van, get in and DING achievement unlocked.
Heres a short cliff note version incase you want a quick glance through after reading...
* Enter vault and take as much money as possible
* Grab extinguisher on way out
* Throw the extinguisher into the criminals on top of the platform taking cover
* Blast the remaining ones by the train area
* Kill remaining stragglers (if there is any)
* Loot bodies
* Run up right most stairs and make an exit
* Run towards van and get in!
And as the previous poster stated, there is a total of $4'000'000 in every round.
I hope my guide helped you and thanks for reading