Dear Sirithron Kang,
Thank you for your question!
We would like to inform that at present we can only provide Vietnam visa on arrival (Visa to be picked up at the arrival airport in Vietnam and applicable for those who travel by air only).
In your case, you are suggested to come directly to Vietnam Embassy/ Consulate where you are living or staying to obtain your Vietnam visa there.
You need to take your passport, a passport-sized photo, and cash in person to the Consulate, where you will be interviewed about the purpose of your visit. You must also pay the stamping fee and wait in a few days to get the visa. Please note that the application procedures and fees may vary among countries and embassies/consulates.