Listening for a hiss, a visual inspection or the old suck test on likely vacume lines, will probably work better than the spray. Most of the vacume line leaks, I´ve found, were with my eyes or my fingures, during a systematic check of most all of the lines. A finger run along the bottom of the vacume lines, will let you feel chaffs, cuts or rotten rubber. Some day I´m gonna buy a hand vacume pump. I´d pull a vacum test (suck test) on the vacume lines running into the passengers compartment, with the heater controls in the defrost position, will probably help you narrow down the problem. If it´s affecting engine idle much, it´s a fairly large leak.
Most of the vacum leaks, I´ve found were at the plug for the vacume lines at the heater controls. The vacume lines running to the vacume resivoir/canisiter (down the right side under the battery) or along the firewall. Sometimes the plastic lines on the motor itself.
I´ve seen numerous intake manifold leaks, but they rarley affected the heater controls and seemed to change with motor temperature.
Most of the vacume leaks I´ve found, didn´t cause the motor to remain at a high idle, but it would periodically idle up high and the RPM´s would change some at idle (up and down).