Hi,
You need to change the your code as below:
Dim oVendordDOM
Set oVendordDOM = XDocument.GetDOM( "VENDOR_BASIC" )
//Set the property
oVendordDOM.setProperty(“SelectionNamespaces”, “Give Secondary DOM namespace”)
Dim myNode
Set myNode = oVendordDOM.selectSingleNode("//d:VENDOR_BASIC[@VENDOR_ID = '<someid>']")
Dim vendorId
//Get the drop down list node using complete xpath of main DOM
Set vendorId = XDocument.DOM.selectSingleNode("Give complete Xpath of the drop down list box")
vendorId.text = myNode.text