You've learned the recipe to make slugs for the Vending Machines out of Scrap Metal and the acid from fission batteries. Craft them at the Workbench and spend them just like Chips!”
It is received from Christine as part of her dialogue: several Perception and Intelligence checks are necessary, along with polite responses. To get the proper line of dialogue you need to ask her if she knows what the machines around the Villa are for. A dialogue option to acquire this perk has a Perception check of 6 followed by an Intelligence check of 6.
It is of limited use in the Sierra Madre proper due to the shortage of fission batteries, but it is the most profitable way to gather Sierra Madre chips: a 75-cap fission battery and two 1-cap scrap metals result in 50 chips (1.54 caps per chip), while redeeming a 10-cap pack of cigarettes at a vending machine only results in 5 chips (2 caps per chip), and a 40-cap carton of cigarettes only results in 20 chips (2 caps per chip). The worst case - the cashier on the Sierra Madre Casino floor - will only exchange pre-War money for chips at a one-for-one basis (10 caps per chip!).
The vending machine in Elijah's room in the abandoned Brotherhood of Steel bunker, allows for the purchase of any items unlocked on the vending machines at the Sierra Madre. It can be noted that some potentially difficult to obtain items, such as weapon repair kits can be bought from this vending machine (provided they were unlocked during Dead Money), with Sierra Madre chips - making this perk much more cost effective than crafting weapon repair kits at a workbench. However, the player will receive 100 Sierra Madre Chips and a complimentary voucher (if over 7,500 chips had been won at the casino) worth 1,000 chips. This will repeat every three days and will accumulate.