Buenas,

Parece que el problema puede estar aquí:

Código:
AddHandler Puerto_serie.DataReceived, AddressOf Me.Puerto_serie_DataReceived
Si te fijas, más adelante tienes la función comentada...

Código:
 Private Shared Sub DataReceivedHandler(sender As Object, e As SerialDataReceivedEventArgs)
            'Private Sub Puerto_serie_DataReceived(ByVal sender As Object, ByVal e As DataReceivedHandler) Handles Puerto_serie.DataReceived
Un saludo.