If you travel long distances to detect, it's helpful to have a backup detector (Vanquish) with you in your car just in case something happens with your primary detector (Nox 800).
For instance, I've thought I charged my Nox 800 only to find that the annoying connector hadn't actually connected, and I only noticed when I went to grab the detector at 6AM on my way out the door.
Another example: I once had my Nox 800 in my back seat and I moved the driver's seat backwards - which crushed the shaft, making it unusable. Fortunately I had another detector in the car and could still detect that day.
If you don't drive far to go detecting or don't think you need a backup detector, then there probably isn't a reason to keep your Vanquish. The Nox 800 can do more in all situations than the Vanquish (in my opinion, based on observation of friends with the Vanquish and through some limited usage myself).
- Brian