A few years ago, when I was researching my free online guide to Vietnam (which fell to hacker a couple Decembers ago), I took two trips to the popular Halong Bay. One on a mid-range boat tour, which tended to huddle up with other motor-chugging boats in the same bay. You hear LOTS of complaints about these trips. Our guides were a bit jaded, but the boat was quality and I thought it was fun.
The other trip I did completely on my own. I bused to Haiphong, ferried to Cat Ba, and stayed in a guesthouse, rented a motorbike, got hugged by a former North Vietnamese captain in a cave (who gave me one of my Top 3 Travel Hugs), found secret beaches, climbed a scary watchtower on a hilltop, and kayaked in isolation.