The thought echoes in my head like a command from a bureaucrat's megaphone: It is the duty of the honorable bicyclist to maintain a safe speed and proper distance from comrades while riding in group formation. There are 11 of us, descending an unnamed pass in the northwestern corner of Vietnam at 20 miles per hour. The diesel-dank city of Lai Chau is a few miles back; the valley village of Pa Tan is about 30 miles ahead. It's day four of our trip, and by now, after several days of dodging tiny minivans and flatulent motorbikes and Chinese dump trucks, we've learned that bikes are at the bottom of the vehicular food chain.

So why haven't we heeded our guide's warning and slowed down? Before I can convey this thought to the group, Kim, a 39-year-old Bay Area software marketer riding just ahead of me, realizes she's going too fast, jams on her front brake in a state of pre-hairpin panic, crosses up her mountain bike's front wheel, launches over the handlebars, and face-plants. Hard.

I dismount, yell "Rider down!" and rush over to help. Kim's fair complexion has turned green. Her eyes are closed, lids fluttering, and she's moaning like Beavis coming down from a sugar high. Luckily, she manages to walk away with only a few cuts and bruises. The next day, she'll feel good enough to ride.

We're in Vietnam with Roadmonkey Adventure Philanthropy, a startup travel outfitter owned and operated by Paul von Zielbauer, who lives in Brooklyn and, until recently, was a full-time reporter for The New York Times. Roadmonkey's mission can be summed up like so: Lead a group of paying clients on a killer outdoor trip that also involves several days of Peace Corps–style do-gooderism. This is Roadmonkey's inaugural outing, a nine-day, van-supported 300-mile bicycle tour, followed by four days building a playground at an orphanage where the majority of the children have HIV.

I'll come right out and say it: I had misgivings about this trip. I'd looked into pay-to-volunteer vacations in the past, not just as a client but for a possible career move. "Voluntourism," as it's often called, sounded like a meaningful way to travel and, perhaps, a half-decent way to make a living, at least compared with the one I've eked out as a freelance writer. But I wasn't quite convinced. Volunteer vacations seemed like an excuse for Westerners to catch an adrenaline rush, safely slum it in some beautiful but poverty-stricken corner of the world, and then preen with false humility over syrah and tapenade back home. What could I accomplish that a low-wage Vietnamese laborer couldn't, besides letting others know that I'm an up-with-people kind of guy?

For starters, I could raise money through my own social networks, as each of my fellow tripmates was expected to do. That money would go to one of the Vietnamese orphanage's primary backers, the New Jersey–based nonprofit Worldwide Orphans Foundation (WWO). One e-mail blast, one Facebook posting, and four days later, I'd raised about $800, much of it from people I hadn't spoken to in a decade. Then I caught a flight to Hanoi to see for myself if Roadmonkey's brand of voluntourism was a noble niche within the travel business or a form of philanthro-washing that would leave me feeling unfulfilled.

MAN, THESE KIDS are cute—especially this one little dude, Lanh. (Because Lanh is a minor under government care, I've changed his name.) I first meet him in the kids' sleeping quarters at the orphanage, near the city of Ba Vi, about an hour west of Hanoi. The orphanage, essentially a courtyard with several rooms on its perimeter, was once the health center of the adjacent Ba Vi prison, a low-security facility for drug offenders and prostitutes about 100 yards away. Lanh has been at Ba Vi for about two years. Like all but a few of the 60 HIV-positive kids here, he appears surprisingly healthy. Because he is.