expansion vessel is full of water and the boiler is locked out ?

Your Vessel needs recharging with compressed air to 1-00 Bar, keep a drain valve open whilst pumping up to allow the water to discharge from it, as long as the air charge holds at 1-00 bar, the vessel can be deemed as OK, (it should be recharged at every annual service)
The Lock-out problem is not connected with the expansion vessel, and should be investigated by an oil engineer, it does sound like this boiler has been poorly maintained.

www.elanar.co.uk
October 2009

Definitely you need a new expansion vessel. It should be filled to around 10 psi with air or nitrogen, and when its in place, increase your boiler pressure to around 1 bar.

John
October 2009