I could only recomend Cat ba Ventures. The owner is really helpful, I organised our trip by emails and he was always quick with responses. I had my worries by sending the deposit via bank transfer, as you never know But everything was as he said. His driver picked us up from Hanoi, took as to Cat ba via the ferry, where we payed the rest. Than we went to the hotel he booked for us, and 2 days later his people picked us up for our boat trip which was organised by his company. We had an amazing time on the boat which was just for the 2 of us. The staff on the boat was really nice, we went where we wanted, and when I got ill they were really caring. The boat was fine, nothing fancy but it was all ours which was just great.
If you want a yalong bay trip without the turist thong and the huge boats I suggest to use this company.