As my trike has a non standard system, it has an independant fuel pump which is switched one at ignition turn on, it constantly runs and returns to the tank constantly via the original FPR, in this cenario does the vaccum connection to the FPR actually do anything.

Yes it does. While the operation of the fuel pump may not be standard, the FPR will function the same and that goes for the vacuum tube as well. Without it your fuel will run a little rich under accelereation

Am I to assume from that, that the fuel pressure is reduced slightly under maximum vaccum.

Injectors rely on a constant pressure to deliver the correct amount of fuel. When the vacuum increases in the intake manifold under accelleration, the flow of fuel will increase due to the greater pressure differential between the fuel rail and the injector nozzle. The vacuum tube relays this change in pressure to the FPR which then regulates the fuel pressure to maintai the correct flow thru the injectors. If the tube collapses/kinks or is disconnected, the FPR won't be able to adjust the fuel pressure and the mix will become very rich when accellerating.

Gauntlett wrote:Am I to assume from that, that the fuel pressure is reduced slightly under maximum vaccum.

yes that is correct

the vacuum at the throttle body butterflies take off point reduces as the throttle is opened

if you watch the fuel pressure through the fpr  it pulses to a lower average value at idle  from a static 2.5bar (vacuum line removed )