When the mission begins, walk into the back office and intimidate the employees by pointing your weapon at them (DON'T SHOOT THEM!) to make them shuffle into the back room.
Whilst you're doing that, Trevor sets the charges on the vault door. Once he's back, use your phone’s remote function to detonate the charges.
Follow Trevor into the vault and collect the cash on the ground and in the back right corner.
As you exit the vault, a security guard holds Michael up.
From here, switch to Trevor by pressing and holding Down on the D-Pad and then use the Right Analog Stick to choose Trevor. Once the perspective changes, aim at the guard's head and shoot him.
Follow the boys towards the exit and take cover as Brad detonates the doors. From there, take cover at the edge of the garage door and pick off the cops running towards you in the distance. Gradually advance from pillar to pillar, hugging cover. Cops will try to flank you on your left so watch out.
Then step forward, running and gunning at the same time, (by not aiming with LT and only holding down the RT) go down the hill and then reach the getaway car and hop in.
Be gentle on the wheel, and stay in the right lane to avoid an unfortunate collision with oncoming police or traffic. Stay on the road and don’t attempt any cross-country shortcuts or the mission will end.
Beat the train, and after the car crash cut scene, take cover and defend your fallen comrade.
After several cops are down, the mission is complete.