Another commenter pointed out the factual error about vitamin D coming from IR vs. UV, but there’s another misconception in this post.
The amount of IR you get exposed to is not the same thing as how warm your room is. If you touch a hot pan, or get hot because you’re hanging out where the air temperature is high, that heat isn’t being transferred to you by IR. It’s being transferred by thermal conduction: https://en.m.wikipedia.org/wiki/Thermal_conduction
So even if vitamin D was created by IR, you wouldn’t get any from sitting in a hot room.
The amount of IR you get exposed to is not the same thing as how warm your room is. If you touch a hot pan, or get hot because you’re hanging out where the air temperature is high, that heat isn’t being transferred to you by IR. It’s being transferred by thermal conduction: https://en.m.wikipedia.org/wiki/Thermal_conduction
So even if vitamin D was created by IR, you wouldn’t get any from sitting in a hot room.