A Good forum do not delete post in open threads, a forum full of unuseful threads and dead ends in their post will no one like.
See your self this forum if the 1000k++ post guys decided to just delete them self and their posts, it wouldn't be anything else left than a totall mess
Not sure on where legality comes into it...Are any posts I make owned by me? And therefore reserve the right to remove them?
Assuming I "own" my posts:
Then I reserve the right to delete them all regardless of what the number is, or what affect it will have on the forum. One would hope however that they would not wish to do so of course. Your years of contribution then all becomes wasted effort.
Assuming I don't "own" my posts:
I still own my personal data, and can challenge what is stored about me.
This can be handled while retaining the "value added" stuff that contribute to the forum content - that I don't own.
I'm not familiar with vBulletin, but Id imagine its not rocket science to orphan the posts from the user. So the user can be removed, and the posts remain.
If orphaning isn't an option in vBulletin (database model was designed to be high integrity) another way would be providing the facility to "scramble" all your account data to meaningless crap. Keeping your user alive(albeit useless, with an inability to login) and still retaining "useful content".
Both methods obviously won't cover your privacy if you have shared personal information directly in your posts. At least it will keep the "(useful) content" active that you have contributed to the forum during your tenor.
As confirmed above, mods don't delete accounts. Which could very well go some way towards our reported 48,000 members...How many have long gone? Its just advertising.