My car runs in Group 4 with CVAR and they definitely have rules on carbs. I'm limited to running with the Zeniths that the car came with. Group 4 is the "super vintage" group, meaning cars that are 1961 and older, and they really try to keep this group true to the "spirit" of vintage racing. However, if I were to run in Group 2, which is the regular production group that my car would qualify for, I'm still limited to no more carb venturis than came on the car originally. In other words, a single side-draft Weber would work but dual side-draft Webers are a no-no.
I've a friend named Roger Davis who runs his Series V with RMVR. His car has dual Weber side-drafts and he claims that the RMRV doesn't give him a hard time. I don't think he runs a full schedule with them, though, and according to what RootesRacer said about what RMVR told him about letting him run a few races a year, that may be how Roger is getting to run those carbs. Roger and I have talked about him coming down here to run with CVAR at Hallett here in Oklahoma, but there would be no real class for him. I'm sure as a guest they'd let him run, but probably in the FIA class due to the carbs.
From what I can gather, the boys on the west coast have much more relaxed rules about what constitutes vintage than the rest of the country. As far as I'm concerned, I like the true vintage aspect of it all. Too many modifications raise the costs too much and leave you with a car that's too updated to be truly called vintage anymore.
My .02 worth,