Vending machines are automated sales units which sell a variety of products from different companies.
The machines offer a variety of items to purchase, mostly from one company.
A Hack skill of 20 or higher allows you to sell items to the Vending Machine, and at skill level 40 allows you to purchase restricted items, such as the bypass shunt.
Restricted items on a vendor can only be purchased when you have high enough reputation with the associated faction, or a Hack skill of 40 or higher (requires minimum level 16 to achieve).