With our bikes sold for half the price we bought them (a good deal for 5 weeks travel), we got a bus to Ha Giang for the final leg of the trip. The Ha Giang Loop is a loop of roads near the Chinese border with some of the best views I've ever seen.
We booked into Jasmine Hostel in Ha Giang, who organise rides around the Ha Giang Loop led by local guides. You can ride on the back of one of theirs, hire one out, or bring your own. It was by far the best part of the whole trip. In a group of ~20-25 people we all spent 3 nights on the loop, staying at different hostels and having big communal family dinners every night, helped along by lots of rice wine and beer.
The loop itself was breathtaking. Every corner you took was another incredible view, outshining most of what we had already seen up to that point, and that was just when we were driving. There were other sights sprinkled in between the drives. There were caves and hikes up the mountains to even better views. Occasionally we'd pass through a town and see a market or temple. We also crossed a couple of feet into China where one of the roads went close to the border. There is an enormous Vietnamese flag here atop a tower that (again) gives amazing views.
The driving was not easy, as there were lots of hairpin turns and often very poor road quality, but they were never very fast so even when a couple of people slipped off, they could hop right back on no problems. I'm glad I'd already ridden for a few weeks before going to the Ha Giang Loop.
Overall: the best part of the trip, and I'd recommend going last so that it doesn't overshadow all subsequent views in Vietnam, and so you already know what riding in Vietnam is like (nothing like Europe or NA).