How do I hide certain forums unless a link is clicked (like vbulletin.org)...?

[flup]

What you are after is this modification: https://vborg.vbsupport.ru/showthread.php?t=92201

It works from versions 3.5.4 till 3.7.2 as far as I know. After installing this mod, you can edit your forms and assign a page to them (numeric values), after that, use index.php (or perhaps you named your forum file forum.php) to call the right page as follows:

index.php?page=1 (to display the forums for tab 1)
index.php?page=2 (to display the forums for tab 2) etc.

Ofcourse these links have to be placed somewhere on your website (text/button), I have done the same at my forums www.clubpunto.nl, the buttons on top of the website link to the 'tabbed' forums.