You are always better off with an independent speed sensor if instantaneous accuracy is important to you. GPS drift can cause substantial variation in reported speeds on your computer, particularily worse in areas where a clear view of the sky is inhibited by trees, buildings or terrain.
I agree that a real speed sensor is better than GPS, especially if you’re using a phone’s GPS. And now that there’s Bluetooth support from Vector 3 (and other manufacturer’s power pedals), the latter is a real option.
A fun fact: if you zoom in closely on a recorded ride and see that the location is frequently off the road, you can bet (and confirm) that the GPS track came from a phone. If it tracks the road well, it probably came from a dedicated (and relatively late-model) GPS unit such as a Garmin Edge.
So, for someone who could, say, afford Vector 2 / 3 pedals or a Garmin Edge but not both and went with the phower pedals, a speed sensor would be recommended.
What you don’t need with Vector pedals is a cadence sensor—it’s built in and I’ve heard it reported here (the Garmin forums) that if you have Vector pedals and a cadence sensor, the Edge unit will prefer the pedals for cadence data, anyway.
