В Сессии мне кажеться не очень эффективно такое хранить, но идея правильная

Private ReadOnly Property ViewStateFilePath() As String
            Get
                Dim str_fold As String
                Dim str_file As String
                '// 
                str_fold = System.Configuration.ConfigurationManager.AppSettings.Item("PersistedViewState") '"c:\proc\PersistedViewState\" 'System.IO.Path.Combine(Request.PhysicalApplicationPath, "PersistedViewState")
                str_file = Session.SessionID & "-" & Request.ServerVariables("SERVER_NAME").ToString + Request.ServerVariables("SCRIPT_NAME").ToString.Replace("/", "-") & ".vs"
                '// 
                Return System.IO.Path.Combine(str_fold, str_file)
            End Get
        End Property
 
 

        Protected Overrides Function LoadPageStateFromPersistenceMedium() As Object
            '* determine which file to pull 
            If Not (System.IO.File.Exists(ViewStateFilePath)) Then
                Return Nothing
            Else
                '* open file 
                Dim obj_read As System.IO.StreamReader
                Dim str_read As String
                Dim obj_frmt As System.Web.UI.LosFormatter
                '// 
                obj_read = System.IO.File.OpenText(ViewStateFilePath)
                str_read = obj_read.ReadToEnd
                obj_read.Close()
                obj_frmt = New System.Web.UI.LosFormatter
                '// 
                Return obj_frmt.Deserialize(str_read)
            End If
        End Function
 
        Protected Overrides Sub SavePageStateToPersistenceMedium(ByVal viewState As Object)
            Dim obj_frmt As System.Web.UI.LosFormatter = New System.Web.UI.LosFormatter
            Dim obj_stgw As System.IO.StringWriter = New System.IO.StringWriter
            Dim obj_strw As System.IO.StreamWriter
            '// 
            obj_frmt.Serialize(obj_stgw, viewState)
            '* save file to disk 
            obj_strw = System.IO.File.CreateText(ViewStateFilePath)
            obj_strw.Write(obj_stgw.ToString)
            obj_strw.Close()
        End Sub

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