Due Diligence and Ongoing Monitoring

Once an initial w9 is obtained from the vendor should we request one again after a specific year? 

Requesting a W9 with the documents as part of a periodic review is reasonable and easily provided and shows a current/updated document for the review period. If the Vendor is a lower tier and you aren't conducting a review then asking for it annually is not a bad idea so you have it but outside of a vendor name change that document probably won't change much so base it on what you feel is appropriate for the vendor type and level of oversight you need.