Window.vala

using Gtk;

class Contacts : Gtk.Window{

    public Contacts(){
        this.set_title("Contacts");
        this.set_resizable(true);
        this.set_default_size(1000,500);
    }

    private void pack(){
        Gtk.Stack stack = new Gtk.Stack();
        Gtk.StackSidebar side = new Gtk.StackSidebar();
        Gtk.Label label = new Gtk.Label("Label Qualquer 1");

        stack.add_named(label, "Pagina 1");
        side.set_stack(stack);
        this.add(stack);

    }

    public void run(){
        this.pack();
        this.show_all();
    }
}

public static int main(string[] args){
    Gtk.init(ref args);
    Contacts c = new Contacts();
    c.run();
    Gtk.main();


    return 0;

}