1998 F-150 appears to have "vapor lock"...How to Fix Problem?

"Vapor lock" is a term I haven't heard used for a long time. You are talking about the battery, the starter, and 'turning the lower pulley"? That sounds more like hydrolocked than vapor locked. Vapor lock is an old term used before electric fuel pumps and fuel injection when an air bubble got in the fuel line, and all you had to do was wait 10 or 15 minutes and the car would start. But it would crank over easily. Hydrolocked is a blown head gasket when coolant gets in to a combustion chamber, both valves are closed and the engine is seized. I don't know if that is what happened here, just trying to make the distinction. So what's the deal? It won't crank over when you try to start it? Lets go from there..and you try to turn the engine at the harmonic balancer aka crank pulley and it won't move but a little bit? One thing you can do is pull out all the spark plugs and try to crank it over by hand or with regular starter for just a few seconds...see what happens. If coolant shoots out a spark plug hole it was hydrolocked.

Does your engine look like this? The reason I ask is the V6 4.2 has aluminum heads notorious for warping.

A warped head wont prevent the engine from turning. when you try to rotate the crank,(lower pulley) does it stop like its making hard contact with something or does it stop like its trying to compress? was there a knock when it last ran? pulling the spark plugs will relieve compression and possibly allow it to turn and will allow you to see into the cylinder. you may have "dropped" a valve into the cyl. which will deffinatly stop rotation

It is a "compression" problem...or some other type of "seizure"... Nothing "knocked"... Nothing is "hitting" inside cylinders... Can an AC pump seizure cause this amount of pressure... I will release the belt and see if things are "free" before going deeper... There was NO abuse to the motor... It ran without a "hitch" for 3-hours... Was turned-off... And, will not crank... RPM never over 3500... HOT weather was the only possible cure... All gages were scanned while driving... No electrical or idiot lights... No elevated temps in motor...

if one of the valve break it will be in the cylinder and would allow it to rotate to only certain degree but not all the way. thats one possible cause , it happened to me.