I think the headset is at least as important as the dongle since we are primarily interested in sending data from the headset. The transmission signal strength of traditional BT headsets is bound to be quite low since one of their primary selling points is usually long battery life. To get extended range on a Bluetooth headset you probably need to have class 1 at both ends, but class 1 headsets are rare and expensive, and if you are going to be paying more than $100 then you might as well consider an option with better sound because Bluetooth is still not really a great choice for VoxCommando in terms of accuracy.
That said you can always have a look at something like the callpod dragon headset paired with a decent class 1 dongle. This should provide a whole home solution, even for a pretty large multi-story house.