Chinese Pond-Herons look very similar to Javan Pond-Herons.
In Vietnam, the Javan Pond-Heron only occurs in the south and is present all year round.
The Chinese Pond-Heron occurs in the north all year round and in the rest of the country only during winter.
So, in winter in the south (Ho Chi Minh City, Mekong Delta), both species occur.
Immature and non-breeding Chinese Pond-Herons are distinguishable from the Javan Pond-Heron only by being larger, with a larger bill, and having darker tips to the flight feathers. 
These features are hard to distinguish in the field.

Ho Chi Minh City, Vietnam, 21 December 2023