It's all to do with the settings. General instructions are:
Create a new usergroup called probation or something like that. What I did on IPB was have two groups Member and Members (note the 's' which most don't notice)
People are in one group but promoted with time and posts to the next usergroup when they reach the specified amount. You can also add certain permissions to the first group like not being allowed to send PM's, not being allowed to post in certain forums, have a higher flood limit, have their posts moderated in certain forums etc etc.
That's it in theory. You'll have to play around looking for these settings in the ACP as it's too late here to open it and find them myself right now. Sorry.
If your really struggling to find the settings let us know and I'll have a look later.
Also moving this topic to vBulletin Forum Support and the area it's in now is why it's taken so long to get your first a reply I'd say.
