Standard approved vendor functionality is triggered on inserting the PO line. Depending on the configuration this is either a warning or an error. However, from a user point of view, it can makes sense to also limit the item dropdown to only those items which are approved for the given supplier.

I am aware of the functionality that you can see which items are approved to supply starting from a PO, however, it seems cumbersome to having to do this compared to limiting the item dropdown.