According to the Hierarchy of Controls, the most effective way to prevent and control hazards is to eliminate them or substitute them with safer alternatives. In this case, the hazard is the potential leakage of customer data by the vendor. Therefore, the most effective control would be to eliminate or substitute the customer data with masked or anonymized data fields. This would prevent the vendor from accessing or disclosing any sensitive or identifiable information about the customers. Masking customer data fields is an example of an engineering control, which reduces or prevents hazards from coming into contact with workers or third parties. References = Hierarchy of Controls, 5 Risk Control Measures In The Workplace