What is wrong with this theme at the moment. It is plain and simple the way you would want a forum. You don't want loads of flashy bits otherwise it will get distracting.
Although If you were to get another them I reckon you should have the same colour set and also make it so people can change between different themes as the members will like it if they have a choice.
Also if you were to get another theme you could always make it a completion and then you could get every one who wants to take part make one and submit it and then David or David and the Staff can decide which one they wont. Although there would have to be a prize otherwise no one would enter as it is a lot of work to make a whole knew theme for vBulletin.
But if you were willing to invest on a new theme then you can get one custom made for you at http://www.completevb.com/
If you do not want to make one or have to pay for someone to make one then just go over to http://www.vbulletin.org
And have a look round.
Hope this is helpful : )