I've lived in Vietnam for 4 years now and have traveled extensively by motorbike over the entire country. On all of the roads, there is various livestock roaming freely. Goats, pigs, dogs, cats, chickens, buffalo, etc... they are either just on the street or in pens in peoples literal shacks by the side of the road.
When driving from south to north (towards China), at some point, you stop seeing pigs entirely. There are checkpoints every few km where you are required to drive through either a wet hole in the ground or over a wet burlap sack. At some places, they spray your tires. There are signs in the villages telling people to be careful.
Needless to say, for this reason and others, I've stopped eating meat in Vietnam unless I know it is imported from an expat owned restaurant. Even then, I've dialed back my meat consumption quite a bit. Eating street food has entirely lost its appeal.