You can now assign a vendor to an item in the Price Book and in the Inventory Manager.
To apply a vendor to one item, from the Price Book Menu, click the 'Quick Add/Edit Item' box indicated below and make your changes as needed. Select the vendor for the item from the dropdown menu and save as usual.
To apply a vendor to many items at a time, go to the Inventory Manager from the Inventory Menu. Simply search for the items, select them, right click and choose the vendor from the drop down list.