Sorting Gridview w/o DataSource

[ccshine]

I have a Gridview populated with a DataTable that I store in the Session.
I'm trying to sort the columns, but have only been able to get it to
partially work. When I click on a header the first time it sorts as expected,
but thereafter doesn't do anything. I'm attempting to store the current sort
order in ViewState in order to determine what the order should be on a header
click. Can anyone see what I'm doing wrong?

   Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.
EventArgs) Handles MyBase.Load
       'Put user code to initialize the page here
       If Not (IsPostBack) Then
       Dim dtUsers As New DataTable
 
       Try
           Dim conn As New SqlConnection(ConfigurationManager.
ConnectionStrings("WilliganConnectionString").ConnectionString)
           Dim cmd As New SqlCommand("stp_GetUsers", conn)
           conn.Open()
           Dim reader As SqlDataReader = cmd.ExecuteReader()
           If reader.HasRows Then
               dtUsers.Load(reader)
               Dim dvUsers As DataView = dtUsers.DefaultView
               ViewState("SortOrder") = "lastName ASC"
 
               Session("UsersTable") = dvUsers
               grdUsers.DataSource = dvUsers
               grdUsers.DataBind()
           End If
 
           reader.Close()
           reader = Nothing
           cmd.Dispose()
           cmd = Nothing
           conn.Dispose()
           conn = Nothing
 
       Catch ex As Exception
           Trace.Warn(ex.ToString)
       End Try
       Else
           'Only runs on first page load
           grdUsers.HeaderRow.HorizontalAlign = HorizontalAlign.Center
       End If
End Sub
 
   Protected Sub grdUsers_Sort(ByVal sender As Object, ByVal e As System.Web.
UI.WebControls.GridViewSortEventArgs) Handles grdUsers.Sorting
       Dim dvUsers As New DataView()
       dvUsers = Session("UsersTable")
 
       Dim newSortDirection As String
       If InStr(ViewState("SortOrder").ToString(), e.SortExpression.ToString
()) = 0 Then
           newSortDirection = e.SortExpression.ToString() & " ASC"
       Else
           If InStr(ViewState("SortOrder").ToString(), "ASC") > 0 Then
               newSortDirection = Replace(ViewState("SortOrder").ToString(),
"ASC", "DESC")
           Else
               newSortDirection = Replace(ViewState("SortOrder").ToString(),
"DESC", "ASC")
           End If
       End If
       ViewState("SortOrder") = newSortDirection
       Trace.Warn("viewstate sortorder before sorting" & ViewState
("SortOrder"))
       dvUsers.Sort = ViewState("SortOrder").ToString()
       grdUsers.DataSource = dvUsers
       grdUsers.DataBind()
   End Sub