Here's an idea on how the vending machines can work.
First, the Fixer must on a legit vending machine business. This gets him machines of various dispensing types, from cold sodas, toy in a ball types, candy/chips, sandwiches, etc.
Second, all vending machines have a small amount of space for extra goods for storage.
Third, add a special storage slot that feeds into the normal dispensing channel, then modify the interface (by this time it will be a touch screen with nice graphics I bet)
The Fixer only stocks certain items in certain machines due to the items sold in them.
Soda machines can dispense cans that are hollow and hold anything up to a grenade!!!
Candy machines have packs of ammo (some preloaded in clips!) wrapped in packaging matching the candy.
Sandwich machines can dispense hold-out pistols, polymer pistols, whatever in the nice triangular sandwich package.
Each machine would be set up so that you have to enter a certain preset code to access the sub-menu for the illegal goods, and your credstick would have the cost deducted from it.
As to the security of the goods, the storage comparments would be uber-strong boxes and the machines themselves in high-traffic area with lots of other vending machines.
A PC would either call his Fixer and get the location of a machine that has the goods he needs, or would already have that info (from a previous job) or watnot.
This is my take on them!