I just updated my project from gtkmm 2.4 where I would add a toolbar and change it to vertical via: toolbar.set_orientation();
In gtkmm 3.2 this function doesn't exist and I couldn't find anything about it on the "Changes in gtkmm 3.0" page. I tried placing it in a VBox but Gtk::Toolbar doesn't inherit from Gtk::Orientable so I think perhaps gtk has removed this functionality? Is there a way to get the old behaviour?
With Gtkmm 3.4 i can do (the documentation of Gtk 3.2 confirms) :
myToolbar->set_property("orientation", Gtk::ORIENTATION_VERTICAL);
see gtk/gtktoolbar.c (near line 497: g_object_class_override_property(..., "orientation"))
However, the text of icons appears now. To remove:
myToolbar->set_property("toolbar-style", Gtk::TOOLBAR_ICONS);
Hoping that this can help you.