I wouldn't doubt that vibe coding for addons is so far a disaster. Most of the knowledge that any LLM would get about the structure/logic of XenForo would come from public posts here or random posts on warez forums probably. I've had some times were I tried asking it how to do something in one of the more recent versions of XF but it put in random methods that weren't parent methods on a parent class and much more.
If the LLM's had access to the source code, which to be fair I would think at the least Copilot would if there are instances of people pushing up their entire site to a repo, then they would maybe stand a better chance.
I don't think vibe coding will ever be a real thing for anything but re-creating stuff like doom with TypeScript types. I watched a week long live stream of a few programming content creators, one whom is an ex senior Netflix engineer, where they attempted to vibe code out a game. It took them much longer than needed due to having to fix all of the code given to them.