When traveling, it's generally smart to pack in preparation for rain or any other conditions that may arise. The rainfall in Ho Chi Minh City is roughly less than New Plymouth. You can assume this to be the case over the next 1 days.
You can imagine temperatures to be in the 35 °C range in April. The weather in New Plymouth is generally colder than Ho Chi Minh City's. As flight availability is subject to become more limited as peak season approaches it is ideal to book in a timely manner.