If the Thai passport holders want to apply visa at Vietnam Embassy, they should to know that:
Visa administrative Institution: Vietnam Embassy
Processing time: depends on you how fast you need. But for sure, you should make a call to Vietnam Embassy for details.
Required Documents: passport, money and 1 new envelope with stamp on it and your exact home address in order to avoid losing issues.
How to apply: please take a look at contact information of Vietnam Embassy below
EMBASSY OF VIETNAM IN BANGKOK, THAILAND
Address: 83/1 Wireless Road, Lumpini, Pathumwan, Bangkok 10330, THAILAND
Phone: +66-2-515836/7/8 | +66-2-2517202 | +66-2-2513552
CONSULATE GENERAL OF VIETNAM IN KHONKHAEN, THAILAND
Address: 65/6 Chatapadung, Khonkaen 40000, THAILAND
Phone: (66) 4324 2190
Fax: (66) 4324 1154