How to add vte terminal widget in GTK3?

Here a basic example:

#!/usr/bin/env python

from gi.repository import Gtk, Vte
from gi.repository import GLib
import os

terminal     = Vte.Terminal()
terminal.spawn_sync(
    Vte.PtyFlags.DEFAULT,
    os.environ['HOME'],
    ["/bin/sh"],
    [],
    GLib.SpawnFlags.DO_NOT_REAP_CHILD,
    None,
    None,
    )

win = Gtk.Window()
win.connect('delete-event', Gtk.main_quit)
win.add(terminal)
win.show_all()

Gtk.main()

Building on ADcomp's answer to address wanting to add them dynamically in the future, I think you might want to subclass Vte.terminal.

#!/usr/bin/env python

from gi.repository import Gtk, Vte
from gi.repository import GLib
import os

class MyTerm(Vte.Terminal):
    def __init__(self, *args, **kwds):
        super(MyTerm, self).__init__(*args, **kwds)
        self.spawn_sync(
            Vte.PtyFlags.DEFAULT,
            os.environ['HOME'],
            ["/bin/sh"],
            [],
            GLib.SpawnFlags.DO_NOT_REAP_CHILD,
            None,
            None,
            )

win = Gtk.Window()
win.connect('delete-event', Gtk.main_quit)
bigbox = Gtk.Box()
win.add(bigbox)
bigbox.add(MyTerm())
bigbox.add(MyTerm())
win.show_all()

Gtk.main()

Since VTE 0.38, vte_terminal_fork_command_full () has been renamed to vte_terminal_spawn_sync (). So if you are using newer versions, you have to change @ADcomp's answer into the following:

terminal.spawn_sync(
    Vte.PtyFlags.DEFAULT,
    os.environ['HOME'],
    ["/bin/sh"],
    [],
    GLib.SpawnFlags.DO_NOT_REAP_CHILD,
    None,
    None,
    )