As a point of clarification, the "heater" still gets coolant and is not leaking? BUT the modulation of air flow through the heater core is where the issue is?
Determine where the "blower motor resistor" is, remove it (unplug the connector if necessary), and see if there are any wire-wound resistors which have a gap in their wires? All the fan switch does is change the circuit to determine which resistor (which relates to the fan speed) gets current to make the blower motor run.
For the vacuum issue, there is a small vacuum line which comes off of either the rear of the intake manifold or the power brake booster vac connection. It will enter the passenger compartment on the passenger side of the cowl, then proceed to the vacuum switch on the instrument panel. Usually, at least on a/c cars, if there is no vacuum (for whatever reason), the default mode can tend to be that air flow will come out of the floor vents and defroster vents at the same time. So check for these things.
Those rubber hoses don't always last "forever" or might have chafed against something for a small leak on its side.
There should be a vac hose schematic in the appropriate FSM, similar to a wiring diagram, of the system's vacuum-operated components. Plus the wiring schematic, too.
Just some thoughts,
CBODY67
