I understand your wanting a vaginal birth this time , on one hand ultrasounds are notoriously terrible at predicting baby's weight. On the other hand your doctor also has the experience and education to evaluate the situation for each individual differently than we do. If the baby is that big you might end up with another c section even if you went into labor. A friend of mine was pushing for 2 hours and the baby was stuck, so they went into c section and even then he was stuck and had a hard time getting him out of the birth canal since he already started his way through, he was pretty stuck which can be dangerous. But you don't really know, like the person before said, plenty of women deliver big babies and having some good pelvic stretching positions for childbirth can help I think. If you're just lying in bed with an epidural that doesn't help get out a big baby. You have a tough decision to make!
Going for a VBAC as well. I am 40 weeks exactly today and have it appointment Friday (will be 3 days over due.) The only reason they would have forced me straight to another csection was if I hadn't dilated at all by 41 weeks. Have you had any progress in dilation and effacement? Also, my first was only 7lbs 6oz and this one is not measuring big either. However, many women give birth vaginally to large babies all of the time. My doctor will not induce with medication like pitocin due to it causing stronger contractions that may stress the scar tissue, but if I still haven't gone into labor by Fridaythey will do a cervical ripening (Foley catheter) to see if getting me dilated will jump start labor.
I would suggest looking up more info on your situation... Statistics on birthing large babies, VBAC success rates, the dangers of multiple csections, ect. Ask your doctor for a more informative explanation and tell him/her that you want to know about more options and share your research. Its just as dangerous to have repeat csections and maybe you want to have more children in the future.
Maybe see if you could just call another doctor nearby for an opinion on your situation. Just see if you could explain where you're at in pregnancy, your thoughts, and feelings and then see what the other doctor would recommend. Just to see if s/he is on the same page as your current doctor or not. Some doctors just don't want to mess up numbers, like if a VBAC could possibly turn into a csection they won't risk it because it could end up as a failed VBAC statistic against the doctor...