Private Sub BttnLoad_Click(sender As System.Object, e As System.EventArgs) _
  Handles BttnLoad.Click
        Dim f As New SubForm
        f.TopLevel = False
        f.WindowState = FormWindowState.Maximized
        f.FormBorderStyle = Windows.Forms.FormBorderStyle.None
        f.Dock = DockStyle.Fill
        f.Visible = True
        Me.Panel1.Controls.Add(f)
        f.Show()
    End Sub

Private Sub bttnOpen_Click(sender As System.Object, e As System.EventArgs) _
  Handles bttnOpen.Click
        WorkForm.ShowDialog()
    End Sub

Private Sub BttnDisable_Click(sender As System.Object, e As System.EventArgs) _
  Handles BttnDisable.Click
        SubForm.BttnOpen.Enabled = False
    End Sub

Private Sub BttnDisable_Click(sender As System.Object, e As System.EventArgs) _
  Handles BttnDisable.Click
        MessageBox.Show(SubForm.BttnOpen.Enabled) ' liefert true
        SubForm.BttnOpen.Enabled = False
        MessageBox.Show(SubForm.BttnOpen.Enabled) ' liefert false
    End Sub