The system has to be made HW priority and when heating hot water the weather comp sensor is switched out of circuit and replaced by a potentiometer set to provide about 70 C for cylinder heating.
Tony
I'd suggest; wire it generally as Fig 5.3 (Connection with two 2-port valves) in the VR65 installation instructions, with two exceptions.
The 2 cylinder thermostats are wired just to a 2-port zone valve, adjacent to each cylinder. On a hot water demand, the thermostat contacts close, the valve motors open and the end-limit micro-switch (otherwise not used) closes when the valve is fully open. You'll have three 2-port valves (2 HW, 1 CH).
It is then simply a matter of wiring the two end-limit switches in parallel and connecting back to the VR 65 so that either end-limit switch closing will make the 'CYL' contacts (acting as would the single cylinder thermostat in the diagram) and the HW generation sequence starts.
The DHW zone valve connections in the VR65 are not used; they could be used, but you'd have to ensure they could operate 2 zone valves or drive the valves through a relay. If you don't use them, you won't be able to use the HW time switching in the VRC weather compensation controller.
As stated above, you lose the cylinder temperature indication and the cylinder pasteurisation options by not using the NTC temperature sensor, but that would require a major bit of artificing..
The heating goes off whilst the HW demand is on, so having two cylinders may make this impractical, especially if the house insulation is poor (so that room temperature drops rapidly) or if they're old cylinders (with low heat transfer areas and/or limescale fouling, so taking a long time to heat up).
Didn't Vaillant suggest something like this?