See FSM page EF&EC - 80 for wiring diagram.

(ECCS = ECU in Nissan CA18DET terminology)

The ignitions switch goes into the ECU, there it activates a low-side switch, which operates the ECCS relay. The ECCS relay supplies the actual 12V power to the (rest of the) ECU.
ECU pin 16R/Bswitches on/off the ECCS relay
ECU pins 49,59,109B/W ECU power supply
ECU pin 45B/R Ingition switch, ign. on: 12V
VEMS has no provisions for this mechanism, it just needs its switched 12V, the end. We need to add this simple switching circuit ourselves, like this. All that is needed really, is a switching transistor.
This is a quick and easy way to adapt one system to the other. Probably not the best one, but certainly better than powering VEMS from the Ign.Switch signal directly.
For now, I'll just solder this directly into the wiring, cover all leads in heatshrink, and then ducttape the whole thing. DIYFTW! :)
(Maybe a capacitor for filtering Ign. switch bounce? Gate resistor if a FET is used? TBC).