It sounds like a medical device, in which case marketing it may require FDA approval or notification. Whereas vibe-coding a one-off tool for yourself might still require validation but you're the one taking the risk and accepting liability for it.

I think the thing you're missing is that the tool doesn't need to be marketed because someone else could ask their LLM to make them a similar tool but fitting their use case.

If they're using a 100% vibe-coded tool that they've never read the code of to replace something that would require government approval, for use on real-world patients, they're probably committing medical malpractice as we speak. Let us pray that is not the case.

It doesn't matter if the tool "needs" to be marketed. There is a market of paying customers. If other people are paying $200/month, both your and their lives would be improved significantly by you offering a $100/month replacement software. For all the talk about LLMs replacing the need for packaged software, people are still paying for packaged software, and while they are, you could be making large amounts of money while saving them money. If you're altruistic, you could even release it as FOSS and save a lot of people $200/mo. Unless, of course, your vibe-coded app isn't actually remotely capable of replacing the software in question.