The Hollowrock vault, as the name implies, is found on the map "Hollowrock". This is the final unlockable map for sandbox currently, and the reason for that is likely due to the stack of gold bullion within the vault. Managing to break into this vault, and stealing the gold bullion could provide $2,500. in-addition to that however, the vent entrance of this vault can provide a money bag worth $400, bringing the total amount to $2,900. This is serious money, and can provide many upgrades for your equipment or tools.

To get within the vault, simply locate the warning sign and boxes located towards the backside of the building. You will have to jump into the water from the back of the building. Walk up to the landmass and continue onward until you find a cavern. Close to the entrance, you should notice a grey coloured grill above, this is our entry point. These are both demonstrated by the images below.
You can either destroy, or pull open the vent cover. Doing so will will grant yourself entry to the vent. Proceed to place wooden planks going upwards into the vent, not too steep though! Once you have done this, use these planks to walk up into the vent. Sadly, as the vent ascends upwards, we'll need to place planks either side of the vent, jumping up to each one when placed and repeating this process until reaching the top. This can be claustrophobic, and quite dark even with a flashlight. Either way, your planks should somewhat match mine as demonstrated by the images below.
Once reaching the top, jump and crouch moving towards the second vent cover. Remember to keep crouched, otherwise you could get stuck! The second vent cover may be pushed open alike the first one, otherwise, you can blow it open. And now you're in! Go collect your prize, the well-deserved gold bullion! Make sure to spend it wisely. I decided to blow the interior of the vault to complete rubble using mines, and pipe bombs.

Thank you for checking out my guide, hopefully it was helpful for yourself. If you would like to check out my other Steam guides for various games, they are linked below. Otherwise, you likely should get back into Teardown and start blowing stuff up!

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1929586957
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=381579163
v1.0 - Guide release
v1.1 - Formatting of "Location of the vault" section. Corrected gold bullion to $2,500.
v1.2 - Formatting of guide text. Added information about money bag, plus total cash.