The WYSIWYG editor allows a user to create a new post and see what the end result will look like while the post is being made. This is different from the vBulletin standard editor which requires you to understand BB Code tags.
To enable this editor, you must first make sure it is on in the Admin Control Panel by going to the following location:
Admin Control Panel --> vBulletin Options --> Message Posting Interface Options
Then set the Enable Clickable Message Formatting Controls to be Enable Standard & WYSIWYG Controls.
You and your users will now be able to use the editor if it is enabled in the User Control Panel. To do this go here:
User Control Panel --> Edit Options
And set the Message Editor Interface (towards the bottom) to be Enhanced Interface - Full WYSIWYG Editing.
If you want all of your users to have this enabled, then go here:
Admin Control Panel --> Import & Maintenance --> Execute SQL Query
And from the Automatic Query menu, select Show Enhanced (WYSIWYG) Editor Toolbar then press [Continue].
(Note: In order to be able to do this, your user ID must be in the proper part of config.php. See here for information about configuring this.)
Finally, if you want all new registered users to have the WYSIWYG toolbar enabled, go here:
Admin Control Panel --> vBulletin Options --> User Registration Options
Then go to the Default Registration Options and look for Message Editor Interface. Then set this to be Show Enhanced (WYSIWYG) Editor Toolbar.