You have to serialize a collection of object not only one instance (Config in your case)
Try this :
<Serializable()>
Public Class DataAcquisition
<NonSerialized()>
Public Shared List As List(Of DataAcquisition) = New List(Of DataAcquisition)
Public Sub New()
List.Add(Me)
End Sub
#Region "Déclaration"
Public DateData As Date
Public MesureCapteur As String
#End Region
#Region "Propiétés"
Public Property Dates() As Date
Get
Return DateData
End Get
Set(ByVal value As Date)
DateData = value
End Set
End Property
Public Property CapteurMesure() As String
Get
Return MesureCapteur
End Get
Set(ByVal value As String)
MesureCapteur = value
End Set
End Property
#End Region
#Region "Métodes"
#End Region
End Class
Dim test As DataAcquisition = New DataAcquisition() With {.CapteurMesure = "5", .Dates = Date.Now}
Dim test2 As DataAcquisition = New DataAcquisition() With {.CapteurMesure = "7", .Dates = Date.Today}
Private Sub Button2_Click(sender As System.Object, e As System.EventArgs) Handles Button2.Click
Dim xSerializer As XmlSerializer = New XmlSerializer(GetType(List(Of DataAcquisition)))
Dim writer As TextWriter = New StreamWriter("nomFichierConfigXml.XML")
xSerializer.Serialize(writer, DataAcquisition.List)
writer.Close()
End Sub