The poster will be in one of the safety deposit boxes in the bank vault, chosen at random. The boxes can only be opened with either lockpicking or the saw. Minor spoiler & hint:
SAW STRATS:
Make sure to take the Durable Blade magazine, Fast Motor lower reciever, and Total Ammo boost. Take Ammo Bags as your equipment and have the Extra Lead skill (enforcer list, right hand tree) either basic or aced. Don't grab ammo from your bags until your saw is completely spent. When you're sawing each box, instead of holding left click, quickly double-click each box. That should get it open and save on blade durability.
LOCKPICKING STRATS:
Take the Burglar perk deck and make sure you have at least up to the Luck Of The Irish card. Make sure you also have the Nimble skill (Ghost list, left hand tree) aced.
And of course, in general, having friends to help will make this a lot quicker & easier.