Is the vacuum not just because the top of the thermostat housing is higher than the coolant level in the reservoir? As the brass plug is opened, air is allowed in, from the top, allowing the level to balance out with the reservoir? (or at least to the point, the air above the coolant, in the reservoir, is pressurised slightly, and stops the coolant dropping further in the thermostat housing)...
I'll caveat the above with a big "maybe"... :-)