Perhaps inferred in one the posts above, we walk against traffic whenever we can. While it doesn't prevent a couple of purse or bag snatchers from coming up behind you (motorcycles driving on the wrong side of the street is common in Vietnam), it will help from becoming a victim.
Also very good advice above is to leave the "bling" at home. The only jewelry I carry is a $12 Timex watch.
Don't flash money.
Take the cash you need or anticipate needing for the evening out, especially if you're going to a bar. A lot of pick-pockets work in pairs or a team, often providing a momentary distraction to strike when you've lost a sense of your surroundings.
Another rule I have is to never, ever, order a drink that has brought to you in a bottle or can with the top already open. It should be opened at the table. In fact, I drink straight from the bottle or can and not from the glass. Unfortunately, cocktails served in glasses with ice cubes is off my list too unless I know the bartender or the establishment. I would order a cocktail in a glass with ice cubes at the Sheraton but probably not at some dimly lighted bar on the beach at 10 PM.
Having said that, I do eat street food! My rule here is that it must be served hot! My wife recently ate some cold spring rolls on the street and got sicker than a dog. The salad in the rolls was probably washed or rinsed in local, untreated water. The good news is that doctors are cheap.
Sadly, cyclos are no longer fun and also off my list. As suggested stay away from them unless you're on a tour where the rides were provided as part of the package for the entire group and don't deviate or accept any offers for additional services from the cyclo driver. If you enjoyed the ride and his service, a tip would be welcomed.
Actually, I feel safer in Vietnam than in my own country, the U.S. It all boils down to common sense I guess.