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