Sockets in Vietnam
Type AType CType F
At least one of your the US plug types fits sockets in Vietnam. An adapter is still worth carrying as backup.
Vietnam is the classic trap for American travellers. Its sockets often accept the flat US Type A pins, so your plug slips right in, but Vietnam runs on 220V, nearly double the US 120V. The plug fitting does not mean it's safe.
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For dual-voltage devices yes; single-voltage US appliances can be destroyed by 220V despite fitting.
Only for single-voltage appliances.
Types A, C and F, at 220V / 50Hz.