Uterine rupture. There is a small but extremely serious risk of uterine rupture (the incision on the uterus opening). This is why it is recommended that women vbac in hospitals where there is 24 hour a day in-house capablities to perform an emergency c-section rapidly.
Failed attempt. The major risk with any attempt to vbac (tolac) is failure - this is the 30-40% of cases that require repeat c-section and risk uterine rupture (15% risk) with potentially lethal consequences for the fetus/neonate (and/or the mother). Consider how many more pregnancies you foresee/desire for your family and go with repeat c-section and btl if this is your last pregnancy. Only try at large hospital.