vBulletin Manual - Why are there two Additional CSS Definitions boxes?

The reason for there being two separate input areas is that the first is used by the default vBulletin style, so you are advised to leave the content of this field at its default value so that any subsequent updates made by the vBulletin developers are automatically reflected in your own styles. The second field is not and will never be used by the vBulletin default style, so you may enter any additional CSS you wish here safe in the knowledge that you will never have to revert this field to gain new CSS definitions provided in subsequent versions of vBulletin.

Should you wish to change the values of any of the CSS classes defined in the first Additional CSS Definitions box, you can do so by copying the class definition from the first box, pasting it into the second box and making the appropriate changes there.

As the contents of the second box appears after the contents of the first box in the final style sheet, any classes defined in the first box and redefined in the second will take their final values from the second box.

For example, the first box contains a definition for .bigusername, setting the font size to 14pt. By redefining the font size for .bigusername in the second box, the definition made in the first box is overridden.