As I've said, stock vBulletin doesn't have a problem.
Let me put the analogy a different way; in my webmail hack, there's a mailmessage table with a column, among others, of userid. This column obviously stores the userid of whom the message belongs.
Now if the user was deleted via vB's user deletion system, the user/userfield rows would be deleted and all posts set to guests (not sure how PMs are handled). However, in my hack, the rows with that user's userid would still remain. The results could be unpredicable, from either just junk taking up space in the database, queries failing when trying to join data between tables, or some other bizarre problem.
Virtually every type of hack like my webmail hack--i.e., a hack that stores data per user--is affected by this problem.