air pressure tank in vaillant VC GB 2423 EH gas boiler?

I'm assuming you mean the expansion vessel?
It is located at the back of the boiler, big and brownish red, it has a Schrader valve for replenishing air.
The Schrader valve (which is common to car tyres) is located in one of two places dependant on the age of the model.
Either at the top of the boiler behind the combustion chamber, or more commonly middle bottom of the boiler (surrounded by boiler wiring).

To re-pressure it, you need a car footpump with a guage, release the pressure in the boiler (so it reads zero on the boiler pressure guage), and allow for water exiting the boiler as you re-pressure the vessel.
The pressure should be pumped up to 0.75-0.8bar and the Scrader valve checked for leaks once you have re-pressured and removed the pump.

Carefree
July 2010