Hi Jules
It is possible to define if MRP will create planned orders using the MRP group "creation indicator" on the customizing transaction OPPR. You may create two different MRP groups with different values on the customizing and assign one to the materials with MRP type VB and another to the ones with MRP type PD.
The following blog post provides more details about this setting:
Indicator 'Create Purchase Requisitions' is not considered by MRP
If you want something "automatic" you may consider an implementation of BAdI MD_MRP_PARAMETERS. See BAdIs for MRP for more details about BAdIs on MRP.
BR
Caetano