On Vbulletin admincp, there is the usergroup section, ranks section, and titles section.
Basically i tend to use these and based on members posts # or reputation or a combination of both, the member rank gets upgraded. Example
Can Xenforo 2.x do this ? and automatically update via crons
Post Guard Pro allows you to block invalid or disallowed content submissions using "Post Guards". If a user attempts to submit a prohibited form of content, it will fail and they will receive an error message that you specify for that post guard.
Example uses include:
Allows administrators to set or disable a thread title length minimum requirement on a per-forum basis as seen below.
If the user (of any rank) inputs a title less than the value given, an error will be shown.