Setting "AuthNTLM" property to virtual directory is not updated when using Directory Services

I am trying to set authentication for the virtual directory to use windows authentication.  It doesn't matter what value (i.e. True or False) I assign to vDir.Properties("AuthNTLM").Item(0) it doesn't seem to take and defaults to inheriting the settings from the site.  

Any ideas on overriding the inherited authentication for the site?

Here is my code:

Dim IISSchema As New DirectoryEntry("IIS://" & webSite & "/Schema/AppIsolated")
        Dim canUpdate As Boolean = Not IISSchema.Properties("Syntax").Value.ToString.ToUpper() = "BOOLEAN"
        IISSchema.Dispose()
        Dim IISAdmin As New DirectoryEntry("IIS://" & webSite & "/W3SVC/1/Root")

        If canUpdate Then
            For Each vDir As DirectoryEntry In IISAdmin.Children
                If vDir.Name = appName Then
                   vDir.Properties("AuthNTLM").Item(0) = True
                   vDir.CommitChanges()
                End If
           Next
        End If

Thanks for your help.