I am not sure if it has already been posted, but here is the abstract to
the article on uterine rupture risk in VBAC, which explains the differences
in risk among the 3 types of labor they looked at--
spontaneous,w/prostaglandins, w/o prostaglandins:

Risk of Uterine Rupture during Labor among Women with a Prior Cesarean
Delivery

Mona Lydon-Rochelle, Ph.D., Victoria L. Holt, Ph.D., Thomas R. Easterling,
M.D., and Diane P. Martin, Ph.D.

ABSTRACT

Background: Each year in the United States, approximately 60 percent of
women with a prior cesarean delivery who become pregnant again attempt
labor. Concern persists that a trial of labor may increase the risk of
uterine rupture, an uncommon but serious obstetrical complication.
Methods: We conducted a population-based, retrospective cohort analysis
using data from all primiparous women who gave birth to live singleton
infants by cesarean section in civilian hospitals in Washington State from
1987 through 1996 and who delivered a second singleton child during the
same period (a total of 20,095 women). We assessed the risk of uterine
rupture for deliveries with spontaneous onset of labor, those with labor
induced by prostaglandins, and those in which labor was induced by other
means; these three groups of deliveries were compared with repeated
cesarean delivery without labor.
Results: Uterine rupture occurred at a rate of 1.6 per 1000 among women
with repeated cesarean delivery without labor (11 women), 5.2 per 1000
among women with spontaneous onset of labor (56 women), 7.7 per 1000 among
women whose labor was induced without prostaglandins (15 women), and 24.5
per 1000 among women with prostaglandin-induced labor (9 women). As
compared with the risk in women with repeated cesarean delivery without
labor, uterine rupture was more likely among women with spontaneous onset
of labor (relative risk, 3.3; 95 percent confidence interval, 1.8 to 6.0),
induction of labor without prostaglandins (relative risk, 4.9; 95 percent
confidence interval, 2.4 to 9.7), and induction with prostaglandins
(relative risk, 15.6; 95 percent confidence interval, 8.1 to 30.0).
Conclusions: For women with one prior cesarean delivery, the risk of
uterine rupture is higher among those whose labor is induced than among
those with repeated cesarean delivery without labor. Labor induced with a
prostaglandin confers the highest risk.

Source Information
From the Center for Women's Health Research, Department of Family and Child
Nursing, School of Nursing (M.L.-R.), the Department of Epidemiology,
School of Public Health and Community Medicine (V.L.H.), the Department of
Obstetrics and Gynecology, School of Medicine (T.R.E.), and the Department
of Health Services, School of Public Health and Community Medicine (M.L.-
R., D.P.M.), University of Washington, Seattle.

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