Just thought I'd drop in my view - mickyg and agile are almost certainly right about the expansion vessel needing a recharge with air, these are precharged with air (or sometimes, nitrogen/argon) -see Vaillant faq's for correct pressure and procedure, because water (or any other fluid) does not compress to any noticeable extent. The EV is required on a closed heating system to allow for the expansion when the boiler heats up the fluid in CH system. Basically, the EV is open to the sealed CH system at the bottom and is fitted with a flexible diaphragm, air (or some other suitable gas) is pumped into the upper part of the EV, above the diaphragm (normally something like 0.1- 0.2 bar above usual system operating pressure-but check Vaillant instructions) This precharge must be done with the CH system depressured as per instructions. The air in the top of the EV allows the CH fluid to compress the diaphragm and absorb the extra pressure caused by thermal expansion. The O.P's problem sounds very much like there is little or no air in the EV - if pressure looks about correct when cool (about 1.0 bar) but rises rapidly towards 3.0 bar as boiler heats up CH system, it strongly suggests that the water has nothing to expand against and is operating the relief valve (dumping water out of the closed CH system) which is then causing abnormally low system pressure when the boiler cools down overnight and the water contracts. The relief valve sounds as if it is doing exactly what its designed to do and may well be OK. Get the EV precharge checked first. It is possible that the diaphragm, or the EV itself may have failed, - but check air pressure first, if you get water out of the air valve when you test the pressure, I'm afraid the diaphragm has probably failed. You can test the diaphragm by pumping to, say 1.5 bar with boiler off and cold (preferably with closed system depressured and open to atmosphere), and recheck air pressure after 30 minutes or an hour - drop in pressure almost certainly means the diaphragm has failed. Another clue would be getting air from the radiator bleed valves after putting air in the EV. NB. the EV usually has a Schrader (car tyre type valve) on it somewhere.
Sorry for the long post - Regards, Warrior193.