I'm not sure what idle valve you have. If you have the two wire external, they have a strange setup to them.
Unpowered, (or at 0% duty) the valve is slightly open - to allow the engine to idle.
As the duty % increases, the valve first Closes, then eventually Opens back up and then up to 100%.
I use an IN4007 diode on these type valves and have a pretty good control of them.
It sounds like when you plug the valve in, it is closing and causing the engine to die?
Try holding the throttle blade slightly open, hold the engine at ~1500 rpm or so, then connect the ISV and see if it stays running. If not, then something may be electrically wrong.
Also, there are two spare "injector" outputs in the VEMS, one of which is used for the Wideband O2.
Is it possible to use the other one for the ISV on the V8 cars?