For suburban trains, I don't think a vestibule is really necessary. It's not a bad thing to have, but not really necessary too.
The average journey in a suburban train would be less than an hour long. For most people, even lesser. So there isn't a real need to let people move around the train through the coaches.
And considering that suburban trains run with huge crowds, it is probably a good idea to not have vestibules. Vestibules are just designed to let people pass over them, not for crowds of people...
more... to stand over them. With the kind of crowds we see in our suburban trains, it will be impossible to stop them from trying to crowd around the vestibule area. This will be extremely risky, and most definitely result in a lot of accidents to passengers.