I would suggest you rather then doing from your own take help of any air conditioning repair expert who can look at it and sought out the problem as doing from your own can harm you.
What exactly are you looking for? Generally vac. lines enter the firewall and are used to control HVAC functions (blend doors, etc) behind the controls. It doesn't use vac. to do anything AC related in the eng compartment other than perhaps stop water flow through the heater lines.