OK, Ngo Thanh Nhan just wrote to me after reading the list of place names you want the traditional characters for.  He said:

However, for a start, you may want to ask for a list of Vietnamese towns in Chinese from
 the Vietnamese Embassy in China, or
 the Saigon Giai Phong newspaper or Nhan dan which have a Chinese edition, or
 the tourist books in Japanese, Chinese or Korean
 A Vietnamese-Chinese (Viet-Trung dictionary) that has a list of geographic names

By the way, where did you get the name Càng Long?