Most likely you should be looking into Mackie Control Emulation(currently available in VV5). I own a Tascam 1884 control surface and it works so-so with the Mackie Control Emulation. Judging from how long it took Tascam to get a decent driver ready for Cubase(through the Mackie protocol mind you) I'd say it will be a looong time before anyone sees Vegas-specific protocols from Tascam. I just looked through the 2400 forum also and didn't see Vegas in there once. MCU emulation will have to suffice for you I think.
what's new?
is there any move to better cooperation between vegas and us-2400?
it seems that i pourcased tascam to early since i can adress only first 8 faders instead of 24.
tascam is selectable as 5 midi devices. in vegas i can select only one and after that there is only generic controler to choose.
I could be wrong but I seriously doubt that Tascam or any other audio control surface manufactuer has much incentive to allot their development resources toward a VIDEO app. The only people who understand that Vegas is a decent audio app are current users and there simply are not enough of us to warrant 3rd party developer interest. Its an entirely different story if you're talking about video however.