Keep in mind that there are risks for twin vaginal deliveries that don't exist in singletons -- a big one is the umbilical cord of Twin B beginning to exit before that Twin does -- if that shows any sign of happening, you're probably looking at an emergency C-section.
As much as I didn't want to have a C-section, I really didn't want to have to recover from both a vaginal and a C-section delivery from one pregnancy.
And note that they can turn late. At 33wks, both of mine were vertex. When they were coming out at ~36wks, B had turned breech... (they did an ultrasound in the delivery room to check on them before I got too far along).
mommy to three boys: A, 3/04
and identical twins B and D, arrived 9/08