Hi yes the VR81 is Hard Wired. the only Vaillant units I can find are Programmable room stats like this. http://www.youtube.com/watch?v=TnhT8KUCA_g
I will have a check to see if I can find any compatible thermostats, that are just thermostats.
Found this if its any help. http://www.amazon.co.uk/ST620VBC-VAILLANT-WIRELESS-HEATING-CONTROL/dp/B00C3HIL5Y