@Daehoon ****
yes, it matters. The embassies/consulates have their own local requierments... All visa applications for Thailand are done online, but you have to be in the jurisdiction of the consulate where you apply, i.e. you should be in south Vietnam to apply through HCMC. The jurisdiction for HCMC is: Ho Chi Minh City, Da Nang City, Can Tho City, Thua Thien Hue, Quang Nam, Quang Ngai, Binh Dinh, Phu Yen, Khanh Hoa, Ninh Thuan, Binh Thuan, Dong Nai, Binh Duong, Long An, Tien Giang, Vinh Long, Hau Giang, Bac Lieu, Ca Mau, Ben Tre, Tra Vinh and Ba Ria - Vung Tau.