Although some folks have it working, for most of us the VA11A does not work with Vista. You have choices - run a virtual instance of XP, or spend a couple of bucks on an old XP-capable machine.
However, if you're going to run everything (AHP, 4-modules, the CM15A and the VA11A) make sure you get enough horsepower. As with "Everything X10" logic does NOT rule - Murphy does.
For instance: I bought an obsolete Dell Latitude laptop for $100, and installed only XP (SP2), and AHP. It had only one USB port. I tried a powered USB HUB. I tried a powered dual-port USB PCMCIA card. On idle, the VA11A ate up 80% of the USB bandwidth (internal system report, not just an estimate). Some signals from the CM15A were not getting to AHP. Whether or not it SHOULD WORK, with the VA11A plugged in, AHP was not getting all of the CM15A signals. Unplug the VA11A and no problem (except for that damn popup telling you the VA11A isn't plugged in).
If you really want to run a full-blown X10 system with cameras, sensors, macros, et al, use an XP desktop with multi-channel USB (2.0).
Or wait for x10 to come out with Vista-compatible drivers - (but don't hold yer breath!)
Personally, I run my cameras via Vanguard on the laptop, and everything else via AHP and my Vista desktop. While I don't have the iWitness functionality, everything seems to work.....