For a return style fuel system, I assume the description is correct here and it should be zeroed out:
[ECM] 6950 - Manifold Vacuum Multiplier: This parameter is used to scale inHG to PSI, or to disable manifold vacuum comp. Set to 0 if going to a return style.
Are there any other parameters specific to a return style system (besides all the inferred rail pressure tables and injector pressure multipliers)?