Compatibility summary
You will need a travel adapter in Vietnam; plug types used there differ from United Kingdom.
Plug comparison
United Kingdom plug types: G
Vietnam plug types: A C F
There is no common plug type between United Kingdom (G) and Vietnam (A C F). You need a travel adapter.
Voltage comparison
United Kingdom: 230V, 50Hz
Vietnam: 220V, 50Hz
Voltage difference is within a range many devices tolerate; check your appliance label.