My G6RC 90 series furnace cycles but won't turn on. I was thinking there's something wrong with the "vacuum system" that connects to the gas and burn box. Can you tell me how that vacuum system works?

The inducer motor(exhaust vent motor) has to start up first. If you have a restriction in your furnace flue pipe the switch will not close. Its not uncommon for a restriction at each point of connection of the rubber tube that may need cleaning. Some switches will try to stick. You can remove the hose from the inducer housing or collector box and blow and suck very lightly on the tube with the other end connected to the switch. You should here a click when you do this.