[ericgtr]

You can setup subscriptions here vBulletin Options > User Registration Options > scroll to the bottom and there you can set it so subscriptions are Daily or Weekly. You may also additionally turn this on or off for current users by going to Maintenance > Execute SQL Query and choosing one of the options from the Automatic Query dropdown. By default a user can click on Forum Tools > Subscribe to this forum in any forum they can view.

Keep in mind for this last bit you will need to turn on the ability to run queries in your /includes/config.php file by adding your userid to the line in the section under:
****** USERS WITH QUERY RUNNING PERMISSIONS ******

(just posted the above this morning in another post )

Now, if you want to tie it to a subscription your best bet would be to utilize the Paid Subscriptions. Create a group that has access to those forums and then create a Paid Subscription where users get promoted into that group. At that point they can subscribe to threads.

Also, a simple usergroup conditional can be applied to the subscribe to this thread function if you wanted to leave the forum open to everyone yet only let subscribed members have that option. If you need help with that let me know.