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I had the hardest time understanding air vent drop away pressure. I read @DanHolohan writings about it but it didnt click. Then it did. I hope this video helps you understand how air vents work on a steam system and the drop away pressure.

You can either use the viessmann sensor to run your DHW (most tanks have a sensor well of some kind) or any on/off aquastat switch. the boiler comes out of the box ready for the sensor, with the indirect attached to the boilers dedicated DHW taps. If you are piped with the water heater attached directly to the boiler and just want to use an on/off aquastat control (not viessmann sensor) you need to tell the boiler thats what you are doing via the commissioning assistants on screen prompts. The potential downsides to using a switch instead of sensor are that you won't have access to the DHW temps via the VIcare app, and the boiler will run at max temperature during a DHW call instead of based on demand since the sensor can react to temperature changes as needed. You do not need any 3rd party device (not including the aquastat which you already have) to do any of this. The terminals for DHW are the same whether you use a sensor or a switch, but you need to tell the boiler which you are using, and if you don't have the water heater piped to the boilers dedicated DHW taps you will also need to make that programming change ("upstream" is when the tank is connected directly to the DHW taps on the boiler, "downstream" will not operate the internal 3-way valve and instead the DHW call uses the same flow path as the space heating call)

Then that will cause the individual pipes to get 1/2 flow. That will work better if they feed with 1" using a 1x3/4x3/4 tee.