Imports System.IO
Imports System.net
Imports System.Collections.Specialized
'......

        Dim uriString As String = "TuaPagina.aspx"
        Dim myWebClient As New WebClient
        Dim myQueryStringCollection As New NameValueCollection
'di seguito sono i parametri che passeresti via querystring
        myQueryStringCollection.Add("NomeVar1", "Valore1")
        myQueryStringCollection.Add("NomeVar2", "Valore2")
        myWebClient.QueryString = myQueryStringCollection
        Dim StreamResult As Stream = myWebClient.OpenRead(uriString)
        Dim s As StreamReader
        s = New StreamReader(StreamResult)
        Dim TestoMail as String = s.ReadToEnd
'in TestoMail hai salvato quello che sarebbe stato l'output della pagina aspx