You are 100% correct, pressurized systems that require some sort of external system to stay below pressure are super dangerous for exactly that reason. It’s one of the biggest cost factors in any cryogenic system and is the same issue with designs that account for jacket pressure to reduce she’ll thickness. Last time I checked though, all of the requirements you’d need on an ASME user design specification for a vessel with these features are NOT required if the design pressure is less than 15psig.
It’s also worth noting that even if vessels under those criteria don’t need to be ASME stamped, they still can (should) be.