Since your ultrasound was fine, you probably don't have any extra concern. If you have severe polyhydramnios, I think there is an increase risk of uterine rupture, though I'm not sure how much. Maybe someone else knows.
I had it with one of my daughters, but it was because she had a fatal neural tube defect. I totally don't want that to scare you, because trust me, it is really obvious on any ultrasound. But, regarding the extra fluid, I have to say I had a LOT, and it took about 10 minutes for it to all come out when I was giving birth, but we decided to break the bag in my situation as I'd been laboring (hard, transition labor) for 5 days and was still only 2 cm. She was born 15 minutes after we broke the bag. Of course, we weren't worried about cord prolapse, since we knew she would die shortly after birth no matter what. Anyway, my midwife broke the bag by making a very small opening at first to allow the fluid to exit slowly rather than quickly, while I laid down.
My experience was really different than yours, but at this point I wouldn't worry about it if I were you. Have they told you why they want you to come in right away when your water breaks? Why not wait until labor is well under way? That would concern me more, since it's possible your water could break well before you have active labor, which would jeopardize your VBAC to be in the hospital.
And, I doubt your water would break with your baby still so high. More likely to happen with a lot of pressure on the cervix from baby's head.