That would involve hacking the forum's software and then, because vBulletin issue regular updates, you have to install the hack again after the update.
Quite a few comments!!!! Cool! Anyway, my suggestion is only meant as something voluntary.... we should all police our own threads, marking them as "[RESOLVED]" whether something is a workaround or a hard and fast concrete answer that fixes an issue. (Where would we be without those workarounds ??). And.... regardless of where the threads finally get placed, just re-editing the original post of a thread would really help others to know that something was fixed and that the reply or replies fixed someone's problem or question.
I just think it should be used as a suggestion..... after a while, users will start using it, and policing will take care of the rest who don't mark fixed threads as "[RESOLVED]"
I'd go with greaseman - some kind of suggestion is as far as I would go (don't like rules!). There'd always be people who would forget ignore.
best practice suggestion something along the lines...
If you raise a thread that is subsequently resolved please could members make a final posting - to the effect that the problem has been resolved and a brief description of the solution... (no essay required). This greatly increases the value of old threads using the search facility.