Hi, Andy,
My understanding of the emissions control system is that it only works at idle speed and under decelleration when the ign timing is retarded by a few degrees to reduce NOX emissions. Above certain engine rpm (1700 ?), the control system switches off the vacuum and then you only have centrifugal advance at running speeds.
When certain conditions are met, the control system will switch off the vacuum at idling. One of them is the 100° switch. If the engine is running hot (at 100° C or 212° F) the vacuum will be switched off and the engine idle speed increases to improve cooling.
The switch cam be tested to see if it switches on/off when immersed in boiling water.
Hope this helps.
If my thinking is wrong, no doubt somebody here will put us right.
Naj