Invece che usare WebClient potresti usare WebRequest perchè così riesci a controllare meglio quello che succede

codice:
Dim request As HttpWebRequest = DirectCast(WebRequest.Create("https://urlaction"), HttpWebRequest)

request.Method = "POST"

Dim postData As String = "username=pippo&password=pluto"
Dim byteArray As Byte() = ASCIIEncoding.ASCII.GetBytes(postData)

request.ContentType = "application/x-www-form-urlencoded"
request.ContentLength = byteArray.Length

Dim dataStream As Stream = request.GetRequestStream()
dataStream.Write(byteArray, 0, byteArray.Length)
dataStream.Close()

' Get the response.
Dim response As WebResponse = request.GetResponse()
dataStream = response.GetResponseStream()

Dim reader As New StreamReader(dataStream)
Dim responseFromServer As String = reader.ReadToEnd()
Console.WriteLine(responseFromServer)

reader.Close()
dataStream.Close()
response.Close()
Comunque secondo me non funziona perchè stai chiamando un https (invece che il classico http) e quindi dovresti dare un'occhiata su come abilitare il protocollo SSL

Sergio