The solenoid valve is connected direct to the vacuum reservoir on the back of the engine, without any extra control valves.
If the anti-shudder valve in the inlet manifold works when you key-off (with all the pipes connected) then you have at least some vacuum in the reservoir, it and the vnt control use the same reservoir.
To tell whether it is lack of vacuum or stiffness in the vnt is difficult without measuring at least the vacuum. Could you acquire a gauge? I would give you comparative measurements from mine, but am 300 miles from my car for the next couple of days.