In your Settings.ascx.vb file you should Override the UpdateSettings method and add you validation and redirect on success code there.  Perhaps add some validator controls client-side as a way to reduce server roundtrips for users also.

Public Overrides Sub UpdateSettings()

' Validate 

if [Some condition] then

     ' Display Message

     ' Exit

     Exit Sub

end if

Try
                Dim objModules As New Entities.Modules.ModuleController

                objModules.UpdateTabModuleSetting(TabModuleId, "somevalue", txtValue.Text)

       ' Redirect back to the portal home page
Response.Redirect(NavigateURL("Some URL"), False)         

' refresh cache
                SynchronizeModule()
            Catch exc As Exception           'Module failed to load
                ProcessModuleLoadException(Me, exc)
            End Try

Response.End()

end sub