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 similar to Xian Xianyang. You can assume this to be the case over the next 2 days.
We forecast the temperature to be around the 35 °C's in April, on average. The temperature in Xian Xianyang is generally colder than in Ho Chi Minh City. Book flights in a timely manner in correlation to your departure date.