The Vendor Name field in the Items sublist might be used for selecting different vendors for each item.
Upon checking, I can also confirm that the field is not available in the Custom Form settings so it cannot be hidden by configuring the Custom Form. The following related Enhancement Request has been filed for the possibility to hide the field in the Custom Form:
Enhancement 90617: Item machine shows “Vendor Name” field in view mode of a Purchase Order, even if the multiple vendor feature is turned off.
However, the field can be hidden using SuiteScript since it is available in the SuiteScript Records Browser.