The cost to get the driver signed by Microsoft is not that much these days -- it was much more 5 years ago -- and unless things have changed since I left I really doubt that's a factor in not producing a new Vista driver for the VA11A. We had been talking about it for a while even back in June 2006, and it's pretty much got to do with the development effort involved.
The functional advantage of using a unique driver is having fine access to the still and video capabilities of the device. Also, at the time that the VA11A (and before that the VA10A) was release there weren't any general purpose USB video capture devices available at the price X10 could offer. In order to offer it, we had to write our own driver. The technical advantage is still somewhat important, though that's perhaps less significant than the importance of having people buy the device from X10 instead of someone else.