I had suspected placenta praevia Dx'd at the about 19 weeks, too, but mine was anterior, and by 34 weeks it was right out of the way, so no barrier to vaginal birth.

A year earlier a friend had the same as you, she was told there is pretty much no chance of vaginal birth, and she accepted that she would have to have a C/S at 38 weeks. Her C/S and recovery went really well, with no complications at all. I don't think she had another scan until about 34 weeks, when they confirmed that it was still covering the cervix totally.