What OS platform?
On Linux: If you run qdbusviewer you'll see the dbus methods available. I don't see one for showing the file dialog, but there is one for opening a URL:
qdbus org.mpris.MediaPlayer2.vlc /org/mpris/MediaPlayer2 org.mpris.MediaPlayer2.Player.OpenUri test.mp3
So from Python:
import subprocess
subprocess.call(["qdbus", "org.mpris.MediaPlayer2.vlc", "/org/mpris/MediaPlayer2", "org.mpris.MediaPlayer2.Player.OpenUri", "file:///home/john/test.mp3"])'
Or:
import gobject
gobject.threads_init()
from dbus import glib
glib.init_threads()
import dbus
bus = dbus.SessionBus()
obj = bus.get_object("org.mpris.MediaPlayer2.vlc", "/org/mpris/MediaPlayer2")
iface = dbus.Interface(obj, "org.mpris.MediaPlayer2.Player")
iface.OpenUri("file:///home/john/test.mp3")
On Windows: Try COM?