Andrea Nguyen has published a great post about safe eating while in Vietnam. I followed the same sort of safe eating philosophy while I was there for three weeks this year, and was sick only for a couple of hours about a week in. And I wouldn't do anything differently next time -- I enjoyed the freedom that I had with pretty much eating anything once I'd set my own personal rules about eating fresh food, hot food, drinking lots of bottled water, and not freaking out too much after that.
I would add to this post by saying that I went into my doctor and told him I was going, and that I was going to eat everything, and that I needed meds in case anything happened. He gave me Cipro, which I never took, and a couple other meds for my travel bag.
I had ice in my drinks a little more often than it sounds like Andrea did, but I had a theory that gin killed any bugs that were in my ice (grin).