There is already a section at the Youmacon site and a thread dedicated to guest requests, and I know the DH for Guest Relations doesn't visit cosplay.com, so you may want to post request there. Please note that Vic had already signed on to do another con the same weekend as Youma, so he is completely unavailable for 2007. Perhaps he could come in a future year, but he will not be in attendance this year.
People like Steve Blum are in high demand, but also do not do many conventions. I do know that Otakon was able to get him to sign on as a guest for this year, so you may want to try and attend that convention to be sure you get a chance to see him! (Vic is also appearing at Otakon this year.)