I have bicycled in Yunnan near Vietnam, but never into Vietnam. China is bicyclist paradise. Unless you feel you really need to bring your own bike, leave your bike at home and buy a local Chinese 28" wheel. Cost around $40.00. They ride like a Cadillac. You can literally get your Chinese bike fixed on any street in any town. Bicycle repair-men line the streets. However, if you have something that is not Chinese standard, you will never find a replacement part.
Chinese drivers are accustom to sharing the road with bicycles and most cities have bicycle roads separate from auto traffic.
Vietnam: Hmmm. An adventure to be sure
Man, if you have an accident in Vietnam, consider yourself dead.
My two close friends (two brothers) were in an auto accident in Vietnam about two years ago. The crowd did not assist. Instead, they robbed my injured friends.
No ambulance or assistance came. The younger and less injured brother had to carry his half-dead brother on his shoulders to the nearest "hospital". He said that there were corpses lined up in the waiting area. The older brother said to his brother who was carrying him, "I am going to die here" and he was right.
The younger brother died later. The Vietnamese doctors noticed that his teeth were missing, but somehow didn't notice that his teeth had been pushed up into his sinus cavity.
Vietnam is an adventure to be sure, but at greater than necessary risk. You can find similar culture, climate, and fun in Thailand - not risk free, but better than Vietnam.
I would bike in China and skip anything risky in Vietnam. If you feel you need to go to Vietnam, book yourself a train or flight to Vietnam and into a good hotel. See the sights, eat the food, buy some trinkets, swim in the sea. Go to some temples.
Remember, it may seem safe. Your friends who have been there will tell you it is safe. But it is not safe. Did you ever swim in the ocean and SUDDENLY find a great white shark swimming around you? Well that is what it feels like when trouble hits you in Southeast Asia. Everything is serene until everything goes to hell. If a local gang bears down on you, the locals will turn their backs and close their doors - happened to me.
Some cops will help you, but a lot of the cops are in cahoots with the gangs.
Travel to Southeast Asia by all means, but travel smart and keep your spidey senses "ON" for danger.