As far as operation goes, there is no difference, however, factory setup has it IGN 12v. If the ECU pin 15 goes to earth when power is off, you'll have power going through the solenoid continuously. IGN power to the ISC and the injectors goes through the loom that goes past the washer bottle, so hooking up to it isn't difficult.
PIN 15 readings:
IGN on (eng off): 12v
IGN on (eng running below 3800rpm):0v
IGN on (eng running above 3800rpm):12v

Make sure the VSV is plugged in when you check readings - I've found unplugging it and taking readings off the plug to be misleading.

I doubt there is a vacuum issue as it has been mentioned that the diaphragm pulls in on startup when the VSV is connected to 12v and earth.

To me it sounds like you don't have a good connection to pin 15.
Did you install a pin and length of wire to ECU plug pin15, and solder it to your line to the VSV ??