I’ll pull out my PSI/PCI manual when I get home and see what I can reference. There were actually several DOT regulation references made during the course of the certification class I went through.
Also, if you want to get picky, the requirement is at least once a year, not only once a year. If something were to happen to the tank, i.e. you’re sliding it out of your trunk and it lands neck down on the valve then it should have another VIP done immediately even if one was done a month ago. Anything that happens throughout the course of a year that could jeopardize the integrity of the cylinder requires you to bring it in for a new VIP.