1998 Dodge Ram vaccuum, machanical. Or both

The tach and blower motor have nothing to do with vacuum. A better suspect is the ignition switch. Specifically, you have to start by inspecting the connector and wires. If the connector body is melted or the terminals look dark, you'll also find two of the wires are hard and don't have their normal flexibility for the first four inches. If you find any of those things, the switch must be replaced and I'll describe the rest of the repair.

If everything looks okay with the ignition switch, we'll have to pick one of those items and connect a test light somewhere in the circuit so you can watch it while driving. That will help us find where there's an intermittent break in the circuit.