Well,
I’m in the process of heating my house for the first time. I’ve to check temperature for “plancher chauffant” (underfloor heating) and I’ve 3 VMB1TS… I know I can change offset and temp is 0,5°C step.
I’ve placed the same thermometer next each sensor and adjust the offset but now value are very disparate --> 20,5°C, 22°C and 19,5°C
I would to now if there is any other settings (hidden or not) and if accuracy (quality) of sensor is good 
You can calibrate your sensors with the function Temp Calibration. You have to choose the sensor to calibrate. Use the location buttun until you have the right sensor, then press and hold the menu button for± 4seconds to open the extended menu. Select then Setting menu and scroll until you see Temp calibration. Then you can give the room temperature of the room. Comfirm with the menu button(short press). Exit with location button. Repeat for he other sensors. See page 64 of the VMB1TC Temperature controller manual.
Thanks 
but what about VelbusKink configuration (in Advanced tab, ‘Calibration Factor’) 
seems to be offset (add or soustract value) but not multiplier/diviser…
I do always my calibration on the TC module.
try this:calibrate yours sensors at room temperature(via your TC module if you got one) and see if you have different values in calibration factor in velbuslink for your sensors.