For the benefit of anyone else wanting to do this. I, too, am using 2 x nest 3rd gens to control heating and hot water.
My setup was slightly more complicated in that I have a low loss header and a secondary circulation pump wired to the SL for all 3 zone valves (u/s, d/s, DHW). With the knowledge of another member @bruce_miranda
I wired my DHW zone valve live (ie. HW on) to the right hand CYL terminal of the VR65. The left hand terminal is 230V and is documented as needing bridging to the right terminal via the cylinder stat. In actual fact, it works just like terminal 4 on the vaillant boilers - any 230V live on the right hand terminal activates HW mode.
I also used a VRC 470f, so my outside temp sensor is wireless.
Thanks again @notorious
for posting about this and for the info you gave re your setup.
When fine tuning the heat curve, what is an acceptable amount of time for the heating to increase the room temp? For example, on curve 1.5, I found the boiler on a very low burn, but the room stat was taking an age to increase 1 degree. At 1.75, it is better at the expense of a higher flow temp (albeit still very low - sub 50s). There must be a methodical way to identify the sweet spot for a particular house.