I waited until about that point to really discuss it with my OB. Here is why: I wanted to make sure that any other reasons that might preclude me from having a VBAC had resolved themselves by then, or at least we might know better about how to proceed. Such as: placenta previa (complete or not) - I think in most cases placenta previa resolves by 30 weeks or so, Breech presentation - while babies certainly can, and have, been born successfully breech vaginally, finding a doctor with any experience or skill with breech vaginal birth can be problematic, and while yes, babies can and do flip head down in the last couple of weeks, most babies are head down by 30 weeks or so IIRC - size of baby -while I absolutely agree that ultrasound assessment of baby size is not an exact science by any means, you should be able to get a general idea if the baby you are carrying is going to be on the larger side or not. Also, gestational diabetes screening is usually finished by this point - and depending on how severe you have GD, (re: insulin use or not) may affect what restrictions any patient would face.
IMHO - in any pregnancy, these are things that would affect your ultimate birth plan and whether or not you would be a candidate for vaginal birth. But for a woman planning a VBAC, I think it's importnant from both the patient and provider side to rule out all the other predictable things that might take a VBAC or TOL off the table.
In my own case, I did wait until around 28 weeks to have "the talk" with my OB. And ran like hell once I did. (she gave me very inflated rupture statistics, insisted on an epidural at 5cm, and said they wouldn't let me go a day past 40 weeks due to "supposed" increase in rupture rates after that mark). Should I have talked to her earlier about VBAC (for the record, they knew from day 1 that VBAC was my goal, but didn't discuss it beyond that until I brought it up) - perhaps, but I doubt she would have answered as frankly until all the other things were taken off the table - baby was head down, placenta anterior but high, etc. It worked out well for me in the end - I had a great doula, OB's that listened to me, and a successful VBAC.
As far as restrictions: most want at least a saline lock/heplock/IV access (whatever your hospital calls it) and continuous monitoring. Depending on your provider, they may or may not want to induce/augment with Pitocin unless absolutely necessary (it does or can increase the rate of rupture) this particular caveat was just fine with me. Also, depending on what your hospital offers or allows, I can see them not wanting any water labor/birth due to the difficulty or inability to monitor you continuously in the water.
Also: in regards to weight: with my first (c-section) baby, I started at 225, gained 25 lbs, and baby was 6 lbs 14 oz. No GD. some higher readings towards the end. but I think that was stress (I went to 41+6). My second pregnancy, I started at 225, gained a total of 10 lbs, most of that in the first 1/2 of the pregnancy, no GD again, again higher readings towards the end, and again baby born at 41+6. She weighed 7 lbs 12 oz. (nearly a pound bigger than her brother LOL) and that was my VBAC.
I am pregnant again, starting weight about 220, and I've lost about 4 lbs. I am 14 weeks tomorrow. I am not saying weight isn't a valid concern, but I don't think it's as valid as the comorbidities, like HTN, GD, etc. And I am shorter than you by 4 inches.
I hope some of this helps.