Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Ho Chi Minh City to Okinawa as you normally would.
The rainfall in Ho Chi Minh City is generally more than the rainfall in Okinawa. This will continue over the next 3 days.
Temperatures around the 35 °C's can be experienced in April. The weather in Okinawa is colder than in Ho Chi Minh City. Plan for potential changes in flight availability as peak weather approaches in this location.