Create a class the mirrors your Json data.
<datacontract> _
Public Class Datum
<datamember> _
Public Property nopol As String
End Class
<datacontract> _
Public Class CP
<datamember> _
Public Property data As Datum()
End Class
Load the objects with data.
Dim oDatum As New Datum() With {.nopol = "Test Values"}
Dim oData As New CP() With {.data = {oDatum}}
Then call the Json Serializer
Using oMemStream As New IO.MemoryStream()
Dim oJsonSerializer As New Json.DataContractJsonSerializer(GetType(oData))
oJsonSerializer.WriteObject(oMemStream, oData)
Dim szJsonString As String = System.Text.Encoding.ASCII.GetString(oMemStream.ToArray())
End Using