Dim mtom As MtomMessageEncodingBindingElement =
        New MtomMessageEncodingBindingElement(MessageVersion.Soap12, Encoding.UTF8)

    Dim https As HttpsTransportBindingElement = New HttpsTransportBindingElement()

    Dim binding As CustomBinding = New CustomBinding()
    binding.Elements.Add(mtom)
    binding.Elements.Add(https)

    client.Endpoint.Binding = binding

<customBinding>
  <binding name="SO_12399887">
    <mtomMessageEncoding messageVersion="Soap12" writeEncoding="utf-8" />
    <httpsTransport />
  </binding>
</customBinding>