Abrir un archivo xml XDocument

Dim doc As XDocument = XDocument.Load("C:\archivo.xml")
Dim xmldoc As New XmlDocument

xmldoc.LoadXml(doc.ToString)

Mostrar un xml en un textbox multilinea

Dim doc As New XmlDocument()
doc.LoadXml(lineaxml)
 Dim sb As New StringBuilder()
 Dim settings As New XmlWriterSettings()
 settings.Indent = True
 Dim writer As XmlWriter = XmlWriter.Create(sb, settings)
 doc.WriteTo(writer)
 writer.Close()
tbrespsii.Text = sb.ToString()

Fuente: https://forums.asp.net/t/1302839.aspx?Displaying+xml+file+in+multiline+textbox