My dd wasn't completely posterior - she was facing to the side and slightly forward, but she definitely wasn't engaged prior to the start of labor. I had one cervical check at 36 weeks, and I was 1cm dilated at that point.
Labor started on my EDD. I had some light random contractions in the morning, and then my water broke in the early evening. Contractions became regular after my water broke, but they weren't very strong and they stayed 5 minutes apart. The two things that got my labor going were 1) Power walking - and I mean arm-pumping, sweat-inducing, power walking through my neighborhood, and 2) laboring standing up with my hands on my knees and leaning forward. The walking got my contractions going, and once they were strong enough, I figured out what position I needed to be in to get her turned (based on what felt most comfortable).
My labor was 7 hours total from my water breaking to delivery. Nothing much happened for the first six hours, but after walking and laboring in the forward leaning position, I felt her turn, then engage, and once that happened she came quickly - so quickly she arrived before the midwives.
So IME, baby doesn't haven't to be OA or engaged for labor to start. And for me, walking and trying different positions worked to get her turned and engaged.
I don't have experience with VBAC (this was my first baby) but I wouldn't despair! It's totally possible you will go into labor even without your baby engaged. Will you have freedom of movement to labor and deliver?