I’m using EEBus from Loxone. Works but is fairly basic what you can do with it. The main issue I have is that it doesn’t support cooling.
Vaillant appeared to promise a Modbus gateway in a press release 18 months ago but nothing is available to the public yet. Although a Vaillant UK rep suggested they are using it in some instances in the U.K, but wouldn’t give me any further details.