the real key to your question is: Where is Chinese taught in Vietnam? And how many Chinese schools are there, and where? And more importantly, why would Vietnamese want to study Chinese?

Traditionally, Chinese in Vietnam was limited to the Chinese minority schools run by the Chinese associations, and the study of classical Chinese in the Confucian schools. Since trade with China was largely in the hands of this "Chinese" minority (some of whom had been in Vietnam for centuries), there was no need to study Chinese, especially after the triennial exams for the Mandarinate stopped early in the 20th Century. Written Chinese hung on as the language of the Hue Court until 1945, when Ho Chi Minh (who had studied and spoke Chinese) made the Quoc Ngu the official script of Vietnam.

After April 1975, the Vietnamese government expelled many Nguoi Tau / Minh Huong Chinese, but once the economy opened up, some of those returned.

My gut feeling is that Chinese is taught in some big city Secondary Schools, and in the larger universities. Given the current dispute between China and Vietnam over the Paracel and Spratley islands, you are unlikely to find much interest in studying Mandarin among the common Vietnamese. But there should be a market among those going into tourism and overseas trade, and that is a specialty market that may already have all the Chinese teachers they need.