I just stumbled over this thread because I am looking for a local remote control for a Viessmann Vitodens too.
It gets a bit criss-crossed after a few posts, but I think most of it refers to Optolink.
For boilers without optolink here is what I would resume this thread to:
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OpenTherm seems to be in the making and involves that the complete temperature/weather control has to be redone.
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Hacking the WiFi is near impossible
Has anyone thought about using bluetooth to remotely control the boiler?
However, here’s another approach to a local remote control: