non saprei...
prova ad attenerti alla pagina asp, traducila pari pari in vb.net:

ASPX
=====

<script language="VB" runat="server">

Dim PayID, TransID, ResCode, AutCode, PosDate, TrckID, UD1, UD2, UD3, UD4, UD5, URL as string

Sub Page_Load()
PayID = Request.Form("paymentid")
TransID= Request.Form("tranid")
ResCode = Request.Form("result")
AutCode = Request.Form("auth")
PosDate = Request.Form("postdate")
TrckID = Request.Form("trackid")

URL = "REDIRECT=http://x.x.x.x/carrello_acquisto.aspx?PaymentID=" & PayID & "&TransID=" & TransID & "&IDTran=" & TrckID & "&postdate=" & PosDate & "&resultcode=" & ResCode & "&auth=" & AutCode
Response.write(URL & vbclrf)
Response.write("Payid=" & PayID)

End Sub
</script>

fai questa prova, ciao