I would like to have a new tag for my forum. I want to have a VIP tag that would only allow members of a private forum to be able to see.
Sometimes my members will post about VIP content in the regular forum. I would like to be able to put that content in the [vip][/vip] tag so that it would not be visible to regular members.
Can anyone help with the code?

if they quote someones reply they will be able to see the content since tags are not parsed when you do a quoted reply.
custom tags can be added using the extratags and docode functions, if you look in inc_func_common the code is there, what you will need to do is add a line to extratags and call your own version of docode which does a check to see if they are a VIP and hide the content accordingly, but it will be more difficult for quoted posts as noted above