Almost all the systems I see here now are closed loop, pressurized systems (with the exception of some cooling tower systems). In the old days we had vented heating, which I assume to you is an open pipe in the attic or through the roof so the system pressure is the weight of the liquid. There would be a tank above the highest radiator to accept liquid expansion.

In the pressurized systems we used to have tanks without bladders pressurized to approx 1bar (for a small building) but the problem with these is that over many cycles the air in the tank would become entrained in the water and the tank would get "water logged", and the air would get to the highest point in the system to get bled off. With an auto water feed set to 1bar and no expansion capacity the pressure relief valve (usually set to 2 bar) would blow and the system would be messed up. Pressure would go way down and the feed would open up to 1bar again.

Almost all systems over here now use the bladder tanks to keep the separation between liquid and air so we then have a much wider pressure operating range. I cannot think of a gravity system installed here post 1940 or 1950 and if they were, they were probably converted to pressurized systems sometime since.

The non bladder exp tank had its port on the bottom so the air pocket stayed at the top. I assume this is the same as the "Pressure vessel" you use? A pressurized system must have a make up system expecially if there is a gas or oil boiler involved where a lack of water with no makeup can result in a big hole in the roof. I'm not sure what your local code calls for but in any pressureized closed loop systems we need a water feed, backflow preventer (to keep any chemicals out of the water supply), a low water cutoff to shut off the boiler and a properly sized exp tank.