Accidents, disasters and fights are not safe to be around for obvious reasons, but from experience as a native the locals tend to be rather curious and any unusual gathering is a good sign that one of those is happening.

Trying to help the injured without expertise can make it worse. If the police is already there, they know what to do, otherwise call for them (113), firefighters (114) or an ambulance (115). Try not to obstruct the road (as we witnesses tend to do ;-).

You might get into political trouble by documenting the event against the canonical narrative, though yuan in bribes is very unlikely to get you anything but confused faces. Official fine for misinformation or causing panic is unlikely outrageous in your currency, given you don't sound like there's any financial burden to casually travel to Việt Nam.

Good press is the number one concern, so I can't think of a scenario where it's worth it to pin a foreigner down for a crime. A corrupted officer would try to get bribes from misdemeanors, e.g. traffic violations, instead.

Once again, it's not an invitation to try to jump in to help. Crowds behave poorly in emergency, or in case of traffic accidents, drivers won't look out for you and can collaborate with you to make a new one. It's best to watch from afar and call for help.