How do I make something visible to only one user or usergroup?
 

Let's say I have user (user id # 8901) and I want a message to be displayed on the forumhome template for that user and that user only.

How do i do it?

I assume it's something like

<if userid=8901> message
</if>

but I'm not sure of the exact syntax.

Please help.

I've done that with forum permissions. Create the forum, create a usergroup just for that forum. Turn off permissions for everyone for the new forum, then allow that one new group to see it. Add this new group to whatever users you want to see it.
I run private forums for multiple users like this all the time. It works.

That's not quite what I'm looking for. I want to change the header for a forum that EVERYONE sees so that only ONE person sees the special message.

I.e. the forumhome template. I want one particular user to see some text .. and only that user.

I think this would work....

I think that worked!

vBulletin 3.7.x built-in "Notices" feature can show specific message to specific user or usergroup.