I'm not sure if this will work for everyone, but it worked for me. After searching for a few hours I couldn't find anything on how to increase the height of the header on vBulletin 4.2. Just a bunch of people asking how to do it...
So this is what I have came up with. It seems that the header size directly correlates with the size of your logo. "vbulletin4_logo.png" Since I am using a header alone and not a logo this is what I did.
I created a transparent image in photoshop that was 1 pixel wide, and the height, in my case 250px high. I saved the image as a transparent .png named "vbulletin4_logo.png" and uploaded it to my images/misc folder in the vbulletin folder in my ftp. And boom after many hours it finally worked.
http://www.theperfectpose.com/forum
Hopes this helps.