Vietnam forum

I don't have any experience with this company, I understand your apprehension with the scam aspect of Halong Bay, there are seemingly a million different options and apparently some come with free rats and no food!!
Firstly I don't know if what you have booked comes with transfers from Hanoi to Cat Ba, but I can vouch for the Hoang Long bus company transfers (big bus, mini bus, boat, mini bus 4hrs $11USD) which sounds more daunting than it is. It is well organized, runs 4 times a day and is so cheap! More on that later...
Secondly I don't know what you want to do (junk boat, overnight cruises, kayaking, swimming, rock climbing) but I can suggest you check out slopony adventures, they have a great website, friendly and helpful non-pushy advise on just about everything (probably not a rival company though!!) We found their prices amazingly cheap (they have a base price, then the more people who go the less you pay, obviously you pay more for private tours) And they have a comprehensive step by step of the Hoang Long transfers.
I know that experiences are subjective to personal taste, one man's trash and all that, but we were so happy with them we went out twice, and I can say that we could fault nothing, great staff, plenty of lunch (simple but tasty), street prices drinks, and a casual hassle free feel to the whole day and heard no complaints from any other passengers. You could check them out before you go and see if they offer more than your current booking, especially if you haven't paid any money. Travel safe