If you have a "Vendor No" field on your Sales Order or Sales Order Lines then you have an Extension that put it there. The typical process for a Drop Shipment or Special Order is to set the Purchasing Code on the Sales Line. The Requisition Worksheet then is used to calculate Drop/Special orders. The Req. Worksheet pulls the default Vendor for the Item being purchased from the Item Card, as pointed out already.
You may also notice the Vendor Drop down list in the Req. Worksheet is empty. The Vendor No. lookup in the Req. Worksheet goes to the Vendor Item Catalog, which is established from the Item Card > Vendors action in the ribbon. Once you list available Vendors in the Vendor Item Catalog they will be selectable options in the Req. Worksheet.
