vbModal for a MDI child?

I have a parent MDI form and a child form. The menu on the MDI has a "New Customer" selection which brings up the child form when clicked. How can I prevent the user from going back to the MDI unless they closed the child form? vbModal doesnt work.

Code:

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Private Sub mnuCustomerNewCustomer_Click()

    frmNewCustomer.Show(vbModal)
       
End Sub

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What you may do is disable the MDIForm while showing the MDIChild, didnt checked if it will work though. Or you can still use non-mdichild forms then show it modally.

Yeah, it seems that non-mdi child forms will work better for my program, thx!