Bert,
Now I am more curious than ever. You mention that your forecasts are not generated by SAP, but are generated by an external forecasting program, using requirements type VSF.
This sounds like your interface program is entering Planned Independent Requirements, not forecasts (for VV MRP type, the MRP element for forecast requirements is called "FORERQ" and not "IndReq"). The field to which you refer ("Splitting indicator" in the MRP3 tab) has no impact on Planned Independent Requirements. To my knowledge, it only works with the integrated forecast generation program that is commonly used in VV MRP type.
Best Regards,
DB49