No Question about the vacuum on my '99 3800. So do we just tear it down and inspect the parts? Is there a way to be sure about cracks in the manifold beyond visual inspection?

There is a way to have both the heads and the intake tested to see what is leaking for sure but they will have to be removed. If you can't see anything visually once removed then having them tested will be a last resort.

Is it a strong vacuum? I would first try disconnecting hoses from the crank case one by one to see if that stops the vacuum. There has to be some vacuum to pull the piston blow-by gases back to the combustion chambers, to re-burn those gases.

Only one hose to the crank and that's the PCV and it runs to the intake hose. No manifold vacuum unless they changed something that has been working fine and efficient forever.