I don't know how many people know about this feature or not but since I only discovered it a week ago and wish I had found it a long time ago I figured I'd share it with you.
Some of you have a lot of PM's I'm sure (I have 5000+) and the more you have the harder it is to keep organized. vBulletin doesn't offer threaded PM's like other forum software does (a major minus) but they do have a feature to at least allow you to view the previous posts made in a PM without having to scroll through your in and out boxes.
To see the previous posts, open a PM and look directly underneath the quick reply message box for the "Message History View History" text. Scroll over the "View History" part and you'll see it's highlighted - click that and all the previous message history will appear.
I hope this helps some of you.