I fixed my above post to be more clear.
The most important line in my above post is be the last sentence. If things were to get auto deleted, there's no use for having any message box larger than what is reasonable, as everytime you get a message after it is full, the older one will get deleted. The perk of having a larger storage for premier members is now pointless.
With that being said, however, there's also no way to implement that kind of feature anyway from my end and I highly doubt the newest version of vBulletin has such feature built into it. If no such feature exists, it cannot be implemented or integrated into our forum.
You are in no way "crossing the line," so rest assured that there's no harm in asking.
Sorry I couldn't help.