When you need to carry out some major maintenance for your vBulletin forum, it is always a good idea that you inform your forum users before disabling it.You could leave a message on the top banner of the forum and let them know how long the forum will be unavailable, and when can they expect the the forum to be back on and they can resume communicating with other member.
Once you have given the appropriate notice, you could then disable the forum. Here's how:
For vBulletin 3.x
1. Go to vBulletin Options in the admin section.
2. Select vBulletin Options.
3. Choose to have vBulletin to be active or not.
For vBulletin 4.x
1. Login into the administration panel.
2. Select Options under the Settings section.
3. From the list, select Turn Your vBulletin On and Off, and click on Edit Settings button.
4. From the vBulletin Active section, select the Yes, No radio option.
5. In the section Reason For Turning vBulletin Off, you could also put in your own message, or you could just simply use the default message. In this message box, you can put in the HTML text.
Note: Even when you disable the forum, if you have administrator right, you still can access the forum as per normal.