"Once a caesarian section, always a caesarian section" is an axiom that is simply no longer true! Vaginal births after a prior C-section can now be done! Whether or not you will succeed depends primarily on the reason for your first ceasarian section.
You have a better chance of vaginal birth if your first C-section was done for a "one-time" problem, such as breach (feet-first), fetal distress, toxemia, or placental problems.
If you have an unusual pelvic shape, you may also have trouble delivering vaginally. Your doctor must know the type of caesarian section you had to determine if he or she can allow a trial for vaginal delivery.
If you had a vertical uterine incision (not the skin) you will have to undergo a repeat ceasarian section. This type of incision, known as a "classical incision," puts you at a higher risk for a ruptured uterus if you try for vaginal delivery.
It is very important to have good, open communication with your doctor. Most vaginal births after caesarian section will succeed, depending on your particular situation.