Harold Kestenholz
The purpose of the expansion vessel is to take up the extra water as the water heats. If you increase the vessel initial pressure, the water pressure will immediately jump up to the vessel setting due to hydraulic pressure with no place for the water to go until the system pressure exceeds the vessel setting. so your thinking needs be reversed.
Set the vessel pressure at the pressure the system must have to stay above the highest radiation or coil (plus 4 psig for venting.) The water will immediately enter the vessel as the pressure exceeds this minimum cold fill pressure and the tank can then take the extra water until the maximum water temperature and a pressure 5 psig below the relief valve pressure.
You can read more in the Air Purge procedure at: http://www.hydronicnetwork.net
