Script to update purchase price

Howdy

Having trouble changing the value of the purchase price per vendor on an item record via a script running on line validation on a purchase order. Any pointers? It's the bit in red that won't do it's bit. The exact same piece of code that doesn't work here, works in a script on the item record, but I guess that is because it is in edit mode, as opposed to the record being called from with a purchase order. We have multiple vendors enabled.

 function line_val()    {     var pqty = nlapiGetCurrentLineItemValue('item','quantity')   var gamt = nlapiGetCurrentLineItemValue('item','grossamt')   var pprice = Math.round(gamt / pqty / 1.1 * 10000)/10000   var lastpurch = Math.round(nlapiGetCurrentLineItemValue('item','lastpurchaseprice') * 10000) / 10000   var diff = pprice - lastpurch      if (diff != 0)       {     var changeprice=confirm('The purchase price is different from the item record. Would you like to update this purchase price on the item record?');     if (changeprice==true)      {       var id = nlapiGetCurrentLineItemValue('item','item')       var item = nlapiLoadRecord('inventoryitem', id);       item.setFieldValue('cost', pprice); // this works - changes the cost field on the item but not the vendor pricing       <span style="color:'"Red"'">nlapiSelectLineItem('itemvendor', '1');       nlapiSetCurrentLineItemValue('itemvendor', 'purchaseprice', pprice);       nlapiCommitLineItem('itemvendor');