“Ownership” of the vendor master file varies among organizations, though for a sizable majority (76%), AP owns/manages the vendor master. There is a correlation with organization size—the percentage where AP controls is slightly higher than 76% for smaller organizations and slightly lower for larger organizations. In only 2% of organizations is the responsibility shared between AP and procurement. In a few cases, it is handled by a separate group within shared services apart from AP or purchasing.
One reason AP often has the responsibility is because for many companies, a lot of purchasing is not under the purview of the procurement department. AP receives invoices that procurement does not know about (all payments go through AP).
It depends on the company, its industry, purchasing policies and practices, and more. An important point is that not too many people should have access to the file. Good internal controls demand limited access and segregation of duties around the master file.
See Control Fundamentals: Segregation of Duties. Also there are a number of resources regarding good vendor file management in the Vendor Master File section on TAPN.