If all the Water Vapour condenses in the boiler, then the boiler is working as efficiently as it can. (Presumably the manufacturer quoted efficiency)
This is a can of worms.
In central heating mode the boiler cannot condense all the water vapour if the temperature of the water returning from the radiators is above the dew point temperature. In most houses except for the first few minutes of heating the return water temperature is above dew point temperature
For domestic water heating a combi boiler could be fully condensing as the water supply is well below the dew point temperature. However the design of most domestic combi boilers is such that the domestic hot water is heated in a secondary heat exchanger which is heated by the water used to heat the radiators but diverted from the radiators to feed the secondary heat exchanger. Hence the heat exchanger heated by the combustion does not receive the supply water ( below dew point temperature ) but instead receives water returning from the secondary heat exchanger. This is likely to be above dew point temperature.
The test rig used to "prove" the claimed efficiency may be rather different from the real life situation where the boiler is actually installed.