Assuming that I do in fact have a fuel delivery problem, what are the likely culprits which could go from the engine running just fine to not running at all in one trip?

And if it's not either the GP's or fuel delivery, where does that leave me to begin troubleshooting?

I would wipe down the areas where you beleive the hissing may be comming from with soapy water. Crank the engine again and try to find bubbles/ hissing. that could be important, make sure to wipe the interface of the head and the block.

wide open throttle

... or just a good bit of throttle

do the other vacuum driven components function? shut-down and door locks?

Inspecting the vac system should definitely be on your list.

The hissing is definetely in the rear of the vehicle, which is why I thought it was a vaccuum escaping from the gas cap.

Also, this white smoke - is that uncombusted diesel? I thought uncombusted diesel smoke was black?

I don't have the vehicle handy to verify this absolutely but I'm almost certain I couldn't get the rear doorlocks to function. Possibly the hissing I've heard a couple of times was the door-locks?

Is the entire vaccuum system a single system? I would have thought non-engine essentials like door-locks wouldn't be on the same lines...

Also, what do you mean by shut-down?

oh. I thought you meant rear of the engine.

no. White smoke is unburnt fuel in atomized form, and since your engine is not hot, it should not be water vapor. Black smoke is partially burnt.

somwhere, somehow air is getting into the system (possibly). primer pump / lift pump sometimes causes this, rotten fuel hoses...etc.

if it isnt fuel, my next guess would be compression. Drop a towel over your exhaust end (dont restrict), crank the car, then sniff the towel. DOes it smell like fuel? That could be a compression issue.

can you turn the key and the car shuts down within 2 seconds? Vacuum is used to shut down the engine.

All vacuum is supplied by the vacuum pump on the front of the engine, it does go through some mazes, but they effect each other when there are major leaks. Sometimes not.

The whole vacuum system is linked, yes. I have a friend who carries a collection of golf tees to plug leaks in his 300SD to keep the vacuum going to essential bits (like the fuel shut off and brake assist). They're not on the same lines, per say, but they're not isolated circuits.

I couldn't tell - did you rule out GPs? One trick you can do is to cycle them several times in a row prior to attempting to start (2-3 times is sufficient). That will usually get the car started, even if you're 2 plugs down. These cars seem to do OK with one plug down, but if you get 2 or 3 down, even in mild weather, a cold start can be a PITA.

One the fuel system, you can do a quick check by pulling the canister-type fuel filter in the engine bay. There should be fluid in it. If there isn't, you have a problem in the fuel line somewhere. You can test the line from the tank to the engine bay via the primer pump - it's the knurled knob next to the injection pump. You have to unscrew it / twist it loose, pump until you can hear the bypass valve opening (it sounds kind of like a bike pump), then screw it back tight.

Get a cheap LED flashlight from Kragen/Autozone/Napa/[parts store of your choice], they're pretty essential on the road. For ~$5 you can get one that's on the end of a magnetic pickup wand dealie, and I've found those are incredibly awesome to have around. Makes finding nuts (like the little ones on the GPs that like to fall off when you're putting new plugs in, which then hide under the IP) much easier to find.

When I click the key on should I hear a fuel pump turn on like on a gasser? When I turn the key to the on position it is completely silent.

You will hear something, but it is not a pump. There are electrical components being charged and you just hear them humming.

So no light on the dash lights up and there is no buzzer?

Heh, no no. Everything comes on in the cabin, and as I've said the GP's are indeed getting their 12v (though this doesn't mean their glowing however), I just meant I hear nothing coming from the engine whatsoever until I crank it.

On the w126, a vacuum pump in the trunk operates the locks. The balance is run by the vacuum pump on the engine.

Glow plugs are like light bulbs. They do go out suddenly. They work one time then blow the next. My car acted the same way, like I said in an earlier post.

You could also spray WD-40 in to the intake to get it started. DO NOT USE ETHER!!!!