I am also having a problem with a Viessmann Vitodens 100C Compact. It is 3 years 3 months old and I have the original installer having a look at it - but he seems to be having no success. It was working fine until about 3 months ago.
Basically, it fires up and reaches maybe 45 degrees and then cuts out. It can sometimes gradually reach maybe 75 degrees by upping the temperature a degree at a time, but this takes several hours. In the last few days, it cannot even achieve this. The behaviour is the same whether I have heating, water or both on.
Following Viessmann's initial advice, the thermistors were replaced. Also, the system has been power-flushed, a MagnaClean unit added, all valves on every radiator renewed and a new mid-position valve has been replaced (I have a cylinder with immersion heater). Also, a new pump was tried and that made no difference.
Following further advice from Viessmann, the circuit board was replaced. This made no difference.
Apparently, there is someone else in my town (Bristol) that has exactly the same problem for my installer to solve.
Any ideas?