First test the EGR valve itself. Fit a metal plate between the EGR valve and the exhaust manifold. This will prevent any exhaust gas leakage from upsetting the engine vacuum.
The fact that the EGR valve vacuum actuator won't hold vacuum is not of any consequence at idle since the EGR circuit shouldn't be active at idle. With no vacuum signal the the EGR valve, it should stay closed.
I'd like to understand why some M103s have an EGR valve and some don't. I've never heard of a CA spec vs 49 state spec M103. Is there such a distinction?