Another thing to consider: While I am not an expert on reptiles, I can image the daylight savings might be an issue, i.e. when we switch to summer time and back to normal time. In this case the lightning cycle is shifted by one hour from one day to the other which is no natural, of course, and might impact the reptile. So, may be you need a more calculation to normalize the sunDown time accordingly.
EDIT: Another strategy might be the following: Simply use Vietnam sunrise and sunset times as if they were at local time. This way, the light will be switched on before sunrise, but that would never be fore 6:00 am as this is the earliest sunrise time for Vietnam. You may also want to normalize the time for this strategy to ignore daylight savings.