Right, this is where I can see people getting confused.
Max vacume does occur under idle and overrun, but only inside the intake manifold! The engine is basically a big air pump and if you restrict the air flowing into the intake manifold by closing the throttle down, the engine sucks all the air out of it and causes a large vacume. Agreed? OK.
Now when the throttle is closed or mostly closed under idle and overrun, the pipework to the throttle, which I will call the induction system, is not under much vacume at all even though the manifold is. Why not? because you effectivley have a big open end near the filter and little air flow drawn from the closed throttle. So its the complete reverse to the intake manifold. Now think where the FPR pipe is conected as standard.....oh yes...the induction pipe. So at idle when fuel pressure should be reduced to aid the injector pulse fidelity, the vacume pipe to it is under little vacume, or basically atmospheric pressure.
Open up the throttle and the vacume in the manifold spreads to the induction pipework, from which the fpr is put under more vacume and the fuel pressure rises. As the revs increase, the pump sucks even more air, further increasing the air flow through the induction pipework and from Bernoulli's law of fluid dynamics "an increase in velocity occurs simultaneously with decrease in pressure" a decrease in pressure being an increase in vacume!
So to keep the fuel pressure at 4 bar, we need to conect the fpr vacume pipe to a permenent source of vacume. As the manifold is pretty much always under some sort of vacume then thats where we have put it. The downside being that at idle you still have 4 bar fuel pressure, making it harder for the ecu to trim the injector pulse to obtain the best air fuel ratio. This may lead to a slight increased fuel consumption at idle and maybe higher emissions, but Ive had mine on an emissions tester and it couldnt have been better.
Why did we move it there and not leave it where it was? because it was feared that by removing the restrictions in the induction system and increasing the pipe diameter, you could reduce the amount of vacume, leading to a loss of fuel pressure when needed under acceleration. Vocky actually took some measuments with a digital manometer and compare a standard induction setup for a modified one and found the vacume suffered dramatically.