I cant figure out thye fuel and vaccuum lines

It has a vacuum pump driven off the engine, see if it is working.

FAILURENTO PROPERLY RELEASE FUEL PRESSURE CAN LEAVE AN AIR VACCUUM IN FUEL LINE. -6 DEGREES COULD BE WATER FROZEN IN FUEL LINES

I think what you are looking at is a VENT TUBE and not a VACCUUM TUBE

What is the YEAR/MAKE/MODEL of your Truck?

Chevy 4L60E ?

Take a look at this procedure: http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?lang=en&cc=us&taskId=110&prodSeriesId=380679&prodTypeId=321957&objectID=c00006110

The place to start is to check the vaccuum line to the petcock (fuel switch).
Well, make sure the petcock is in the on position first, but then make sure that pesky vaccuum line hasn't come off...if it did, then no fuel comes out of the tank to the carbs!

The easiest way to look for vacume leak it to go to the vents and back track to the vacuum supplier. somewhere you should to hear the vacuum leak somewhere. It amazes me how much of fixing cars is do by hearing and not seeing. Sometimes seeing can miss lead you. Let me know if I can help you and more.

have a look around the brake master cylider that hold the brake fluid should be somewhere around there. if not then look at the top of the engine where the throttle housing is or around the carb, is it a small vacuum pipe?

This might be as simple as checking for vaccuum leaks in th e air hoses, sounds to me like when the revs drop the vaccuum stalls the car. However, could b thee resistor in the distributer. Check timing, there could be a number of reasons, low fuel pressure, plug gap, point gap, blocked fuel filter, idle stop solonoid on carby. Hope this helps, check those vaccuum lines. Good luck

evap system to fuel tank

On your vette I cant remember if it is vaccuum operated or manual cable but either way get a manual for the car that has a/c heat in it . You either have a vaccuum line that broke or a bad diaphram or a broke cable. I am almost sure its vaccuum but check that first then look at diagram and slowly work your way from the engine to inside car. you can also try to listen for it with the car running. if its a cable operated then look underdash for the blend door and follow the cable to the a/c control on the dash . one of the two. if you cant find a diagram google it. does the controls feel any different now when truning them ?