Do you have the Wunda control box, which controls the UFH pump and powers the UFH zone valve? If so you may wire a 'standard' roomstat to it, and power the actuators from it.
No, there isn't a Wunda control box in the UFH setup. I guess removing the VR 81 from my existing setup and using the Wunda control box with non-vaillant thermostats would be an option, but as the VR 81 is wired into the VR 61 I assume the use of the VR 61 for the UFH (current and new) would be completely removed?
I would then have a system where the radiators where working off the VRC 430 and VR 61 and would use the heating curve/weather compensation (essentially my original system before UFH was added), but the UFH zone (for the existing kitchen and new bathroom) would simply be controlled by a couple of third party thermostats.