Vacuum valve control?

Does anyone know source of engine info is used to control the vac valve which controls the boost bypass solenoid valve. It opens the bypass valve when the engine is on over run with boost present, so is it looking at the manifold boost sensor to see when there is vacuum due to throttle closure, or is it using the boost sensor on the S pipe up to the throttlebody which would show a rise in boost when the throttleplate is quickly closed on overrun, or is it a combination of these?
My Aero is showing voltage applied to the vac valve at idle which is then switching to allow vacuum to the bypass valve, I can't see any reason for the bypass valve to recirculate air at idle when there is no boost present.
Or should the vac valve have 12v present at idle and its the valve that's failed, I have ordered a new one anyway, but am not sure if this system is working correctly. The engine boosts to the top of the orange in the first 3 gears and goes like a rocket, but in 4 th and 5th the boost peaks at the top of the white and surges into the bottom of the orange but no higher.
I have replaced the waste gate control valve and everything is as it should be on that system, DIC was replaced 18 mths ago and plugs and fuel pump are new, it just fades out in the top 2 gears which is a pain for motorway overtaking.
Any tips or ideas will be greatly appreciated as I've been working on this a couple of months now as it takes time to get parts sent to Norway.