Originally Posted by oldford

Of course the vacuüm connection on the fuel pressure regulator must be connected to the inlet manifold. This is necessary to maintain a constant pressure difference between fuel in the fuel rail and air in the intake manifold as this is what the ecu assumes. Otherwise it will be difficult to impossible for the ecu to calculate injection times.
When not connected the mixture will be too rich with low loads (cruising at low rpm's/speed) and it will be way too lean when the turbo is active.

An example (with a bit of exaggeration):
Fuel pressure standard 2.5 bar. So, with the intake manifold at barometric pressure (engine off key on, or accelerating without the turbo) the difference in pressure is 2.5 bar. Now assume the turbo boost is 2.0 bar. Without correction the difference is only 0.5 bar - which means there's not a lot of fuel going into the engine.
Theoretically with turbo boost at 2.5 bar or more the pressure difference is zero or negative, which means NO fuel with get into the engine, but rather air will go into the fuel rail.