anyone ever design a forum theme? is it any different then designing a site?

how would i go about designing it?

(lets say for vBulletin)

I've designed 2 for my own vbulletin forums, and one for Invision Power Board 1.2, i have never designed anything for commercial purposes.

the structure of vBulletin is slightly different from Invision Boards and phpBB

The last time I tried phpBB's it envoled editing like, 40 html files.

Yes, I agree, skinning vbulletin is much easier than skinning invision board .

I no longer own a VBulletin license, so I wil not be able to develop my own right now, if you have vbulletin, i would be willing to help you develop a vbulletin skin.