Question on Views and ViewModels

Some people say that you should have one viewmodel per view. But I am wondering if that is always true. For example, you have a SalesInvoiceView which is divided into the upper portion of the form for the header information and the lower portion of the form for the detail lines in a grid. The SalesInoivceViewModel class holds an observablecollection of salesInvoiceLines which fill the grid. Next to the grid is Add/Edit/Delete buttons which pop up a window for the user to enter the detail lines etc in a form called SalesInvoiceLineView. I think that that this view should be passed the parents viewmodel and both views should share it.

I agree. The SaleInvoiceView should have a SalesInvoiceViewModel but does the SalesInvoiceLineView need a separate ViewModel or can it share the SalesInvoiceViewModel since the SalesInvoiceLineView is a child view of the SalesInvoiceView and can't exist without it.