Microsoft has not required anyone to write new drivers for Vista, and unless a company wants it to be certified "Works with Vista" or "Vista Premium" there's no need to even get an existing driver signed again. Any driver signed for Windows 2000 or XP is supposed to work fine under Vista. So, the WDM model does work in this case, with one exception: unsigned drivers.
Unfortunately, that one exception is where the problem with the VA11A falls. X10 never had the VA11A driver signed by Microsoft (for a couple valid reasons) and the VA11A driver can't be signed in its current form. In order to submit it for signing, X10 will need to rewrite parts of it.