Hi,
Can't really give explicit advice unless I know why you had an EMC previously. You say labour was 56hrs, which implies perhaps an obstructed labour? That would certainly lead to an EMC. Your outward size usually has no relevance to your ability to deliver vaginally. It is more down to your internal os size and whether the baby has a large presenting part, which could lead to Cephalo Pelvic Disproportion, which in English means the baby is too big for the hole!
One thing that being overweight can cause is pregancy induced diabetes and there is a known risk factor of diabetic mothers having larger babies, which in turn is a risk factor for CPD etc etc, catch 22 situation.
Now is definitely not the time to diet, but you should aim for a balanced healthy diet. There are loads of books in the shops and libraries which give ideas for healthy diets for ladies in production. Try to do some gentle exercise, such as pregnancy yoga classes or Pilates has been recommended to me many times. The NCT is quite a useful source of information on this subject.
I'm making the assumption that you're booked for a hospital delivery. Have you checked whether any of the midwives are VBAC trained, as not all of them are. Both of my godchildren were delivered at home, in a birthing pool by VBAC trained midwives [same 2 both times 4 yrs apart!], but then again, my best friend paid privately to engage them.
In the meantime, think carefully about the sort of birth you want and write out a birthing plan, making sure that everyone knows exactly what steps you want taken, if things don't go according to plan. It sometimes helps to have someone other than DH, who can act as an advocate for you. This could be a friend, family member or an outsider such as a birth doula. It might be worth thinking about. The most important thing to remember and to remind your care staff about, is that every pregnancy and delivery is different and they shouldn't make assumptions about this delivery, based on your previous obstetric experience.
Hope this helps, good luck.