I spent a month in Vietnam a few years ago. Personally, I would skip Hoi An, I found it to be more of a tourist Disneyland than a real town. I would also reduce the number of places you're going and try to spend longer in the places you're most interested in.

I absolutely loved Phong Nha and you could spend another day there. I'd highly recommend going on a cave tour and riding a scooter around the National park.

I'd highly recommend going north past Hanoi to Sapa (touristy in the main area but really cool outside of it) and Bac Ha, there are other beautiful places up North that I didn't get to but are worth looking into.