There is apparently some climbing near Ninh Binh.. which is in the northern part of the country .. not far (cpl of hours from memory) to one of the ports you leave to go to Cat Ba from..
Cat Ba is indeed awesome. My only pointer is to watch the tides... In that it needs to be a good (extra) high tide for a lot of the DWS (apparently).. I was there for 5 days, when I turned up I discovered the tides wouldn't really be appropriate for most of that time.. was recovering from a busted shoulder (and recovering from a fever!) and got some sport climbing done at Butterfly valley and a cpl of Islands in the area.. so didn't bother me too much (not to mention I'n not exactly a gun boulderer!)
The tides need to be 'extra' high so the landings aren't too shallow, and as some of the starts are quite undercut with not much in the way of holds below the waterline. The Slopony guys had a chalkboard with ratings for each day coming up in respect to tides. They also had a book listing the tides for the next year, with good days marked, so it may pay to contact them re: best dates to visit.