Where is the AC vacuum port in my 2006 E450 V10?

Looking at the Ford FSM, it looks like the vacuum source is near the center of the intake manifold on the driver's side.

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.

I did a quick search for Vacuum port V10. Got some hits that show pictures and other info. Look like it comes off a common manifold.

For as much as I look at the engine, I can figure out where the vacuum port is in the intake.

Sorry, no. I'm looking for the vacuum port on the engine that feeds the HVAC control system

Sounds like he is looking for the low side of the AC to possibly add freon.

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.