If we subcontract special processes, is it acceptable to ONLY have a certificate of conformance/compliance from the vendor to meet the requirements of 7.5.2?

It depends.

If the subcontractor is able to measure their process output, then it is not special to them (only to us because we can’t).  Therefore, a cert is acceptable and we prefer that it includes data as evidence of actual measurement.

If they are registered to 9001, AS or NADCAP, then we only require their ISO/AS/NADCAP cert because their QMS should meet the requirements of 7.5.2 if they cannot measure their process outputs on the product.

If they are not registered to 9001, AS or NADCAP and they cannot measure the products, then we need to audit them or review objective evidence of their system that they provide to us that might include work instructions, training evidence, process validation, etc.

So to answer the question, through supplier evaluation and approval, we will know which situation exists and therefore, we will know if a certificate is acceptable as all that’s required.