>>24　かなり冗長でテストもしてないけどこんなのは？

Private Sub VewSubDisplay()
Dim monitor() As System.Windows.Forms.Screen
monitor = System.Windows.Forms.Screen.AllScreens()

Dim length As Integer = monitor.Length

If length > 0 Then

For i As Integer = 0 To monitor.Length
If monitor(i).Equals(System.Windows.Forms.Screen.PrimaryScreen) = False Then

Dim rect As Rectangle = monitor(i).WorkingArea
Me.Bounds = rect

End If
Next

End If
End Sub