There's a thin plastic tube coming up the back of the engine, with rubber hose at the end connecting the tube to the rear of the throttle body, what is its' function? It's badly dry rotted and cracked, what effect does that have on performance? I haven't traced it further than I can actually see it so, not sure where it comes from. Yes, I'm replacing it but not sure what it does... I assume it is a vacuum line.

 

Sounds like the vacuum harness. This spiders out to the EGR, fuel pressure regulator, heater, vacuum reservoir and more. This is essential to proper operation of the van.