By code, when I create a new blank site and replace the welcome page layout by a new layout page, the original quick launch menu disapears. How to keep the original quick launch menu of a page even after replacing the page layout by another one?
You must set the master page to one with the quick launch in the ContentPlaceHolder with the id PlaceHolderQuickLaunchTop
<asp:ContentPlaceHolder id="PlaceHolderQuickLaunchTop" runat="server">
Visit MSDN to read more about Creating Pages for SharePoint