MS Access: How to automatically fill in ID field based on value of another field
Assuming that you know how to edit and run a SQL query in Access, I think you need to execute a SQL Query similar to the following:

    UPDATE FoodPurchases
    INNER JOIN Vendors ON FoodPurchases.VendorName = Vendors.VendorName
    SET FoodPurchases.VendorId  = Vendors.VendorId


I have not verified the above, so let me know if it does not work.