Vietnam is both very safe and unsafe: "On the one hand, physical assaults on travellers are rare … the country's terrorism risk is extremely low as well … but petty street crime, deadly roads, dodgy tour operators and Ha Long Bay boats all pose safety risks." (Mark Bowyer, Rusty Compass)
The tap water in Vietnam is not safe to drink: “Boiling water won't remove arsenic and other metal contaminants so stick to bottled water, avoid ice in your drinks and if you're particularly concerned about tummy bugs, avoid salads that may have been rinsed in tap water.” (Choice)
Traveller's diarrhoea is common: “Wash your hands regularly and opt for fresh, fully-cooked food … carry anti-diarrhoeal medication with you as a precaution.” (Choice)
Mosquito-borne diseases including dengue fever, malaria and Japanese encephalitis are a risk in Vietnam: “Consider a vaccination against Japanese encephalitis, and if you're travelling to remote areas, talk to a doctor about malaria prophylactics … there's no vaccination against dengue fever -- avoiding mosquito bites is the best protection.” (Choice)
Penalties for drug offences are severe: “Under the Vietnamese penal code, a person caught in possession of even a small amount of heroin can be sentenced to death … there are over two dozen foreigners currently serving life sentences or facing the death penalty for drug trafficking offences …travellers should be aware that recreational drugs available in Vietnam can be extremely potent … party with caution, and always make sure someone knows where you are and what you have taken.” (World Nomads)