Hello My name is Ian ~ I am not on the site constantly so please be patient whilst waiting for responses to you queries
Was the new thermostat fitted by a Vailant engineer or a local plumber?
My Dad following the very simple instructions supplied with the thermostat and following the suppliers installation video.
I can't see the full question you asked about checking the set back temp, but I don't even get as far as checking the set back temp.
What I meant by checking the temp was to check the room temp which you do simply by pressing any button to activate the screen (goes blank when not in use to save the battery). This shows the display and then activates the boiler without going into any settings or pressing any further buttons.
ie. simply by turning on the screen to read data also turns on the boiler.