Notes for myself, but sharing it in public.
UPDATE prefix_thread SET iconid = 21 WHERE forumid = 12
In this query, I'm replacing all threads in forum 12 with icon 21.
Each forum/subforum in vBulletin has a forumid, which can be seen in the Forum Manager in admincp. Hover over the forum name, and in your status bar, you'll see
The icons for threads are the same as icons for posts. Icon 21 is the ID# given to the icon, as added in the Post Icons Manager in admincp. When hovering, you'll see (somewhere in the URL string)
Note that the table "prefix_threads" will need to be changed to match the prefix for your forum.
Queries are best run from a program like SQLyog Pro for $99
. phpmyadmin sucks by comparison, as do most free MySQL tools -- including the official MySQL interface. SQLyog makes life easier. I need to spend time wisely when working on websites, and SQLyog helps me do that.