Equipment needed: Blasting charge & boltcutters, both of which can be found at rebel outposts/the blackmarket [not dodgy daves]
How to do it: The vault spawns in in random rooms in the HM treasury building within the compound (the building you are looking for is the long office-block). Be aware that as soon as you enter the compound, all cops are alerted that presence there has been spotted, so make sure you find the vault beforehand so you know which doors to boltcut - the vault looks like a big red crate, and will be visible from hte outside of the compound as well. To boltcut into a door, press y, select your boltcut and press use item when next to the door. You may need to boltcut through one door to reach the vault, however only one boltcutter is necessary. Once you've breached into the vault room, you can now place the blasting charge - to do this, press y, select your blasting charge, and select use when looking at the vault. A timer will start ticking on your screen; once this timer has ticked down to 0, the bomb will explode (no damage to you or vehicles, purely aesthetic) and the vault will open. To access the vault, scroll on it, and press spacebar when the option to look at the inventory comes up. Transfer the gold to your vehicle using the 't inventory'. Once you have the trucks filled up, head over to either the Kavala gold buyer (and be prepared to be robbed by us), or the UNMC gold buyer (be prepared to be taxed heavily in most cases), and sell your gold!