Yes, this is a belief popular among certain politically active Vietnamese living overseas. Is there evidence to support it? Yes, if you take a very deliberate reading of history. Is it reliable or convincing? Certainly not.

A Vietnamese professer teaching in Taiwan named Hồ Tuấn Hùng has popularized this theory in his 2008 book Hồ Chí Minh Sinh Bình Khảo. Why has it gained traction? Because the current tension in the South China Sea / East Sea as well as contentious issues related to Chinese bauxite mines in Vietnam has brought increased attention to the Vietnamese Communist Party's close relationship with the Chinese Communist Party. They are tapping into this anti-Chinese sentiment both among Vietnamese in Vietnam and abroad.

According to them, there is the Hồ Chí Minh/Nguyễn Ái Quốc that was an active revolutionary in France and southern China until the early 1930s. Hồ was arrested by the Chinese Kuomintang government and while he was imprisoned fell ill with tuberculosis. Rumors of his death appeared in both the Vietnamese press in Vietnam and France. You can find headlines declaring 'Nguyễn Ái Quốc is dead!' during this time.

So, these authors say that a Chinese imposter was the Hồ Chí Minh was the one that led the DRV government after 1945. They call him Hồ Chú Ba Tàu, which is difficult to translate into English but essentially means the 'Chinese Hồ'.

After Hồ Chí Minh was freed from jail in 1932/1933, he went to study at the Comintern in the Soviet Union. However, the above author says Hồ is already dead. Instead an official named Vera Vasilieva took a Chinese man named Hồ Tập Chương and trained him for five years to become a perfect copy of the real Hồ.

They use this theory to explain the Vietnamese relationship with Mao and communist China. It is true that Ho and the communist party were subordinate to the Chinese, and carried out a bloody land reform campaign based on a near copy of Chinese land reform. Chinese advisor controlled many parts of the program in Vietnam. But the explanation for this is that Hồ and the communists very much wanted membership in the communist bloc and the support they could offer them in their fight against France and the non-communist State of Vietnam. Land reform and other programs were necessary to join Mao and Zhou Enlai (who had to vouch for Hồ and the Vietnamese before Stalin fully accepted them as allies).

There are other myths about Hồ, both ones intended to valorize or denounce him. One example is that communist party historians say that Hồ led peasant tax protests in 1908 and was thus expelled from the Quốc Học school in Huế, before fleeing the colonial police and going to Saigon. This is false. Hồ was not even enrolled during the protests. Conversely, non-communist Vietnamese popularized a theory that Hồ had betrayed Phan Bội Châu to the French police to earn a monetary reward. Though Hồ and Châu were rivals at this time, the evidence all points to someone else having betrayed Châu. You can probably guess that authors ideologically opposed to Hồ and the Vietnamese Communist Party do not care for that evidence. They continue to say that Hồ betrayed Châu.

Another reason why this theory is accepted by overseas Vietnamese (not by historians, though) is because there is and was a cult of personality around Hồ. Many myths and larger than life ideas were propagated by the party. Still today students in Vietnam must study Hồ Chí Minh Thought is schools. So, for example, despite the party saying Hồ was a celibate figure devoted totally to the nation, he had several wives. Yet a professor in Vietnam who mentioned this recently was demoted as punishment. You can see how people having learned about fabrications like that are quick to embrace any theory that breaks down the cult of Hồ and the party.

TL;DR: Hồ was rumored to have died in China in 1932. He did not. Vietnamese writers ideologically opposed to the communist party use this theory to discredit the party and--midst tensions as China expands its presence in the South China Sea--to heighten criticisms of the Vietnamese communist party's close relationship to China.

For the best book on Hồ in this period see Sophie Quinn Judge, Ho Chi Minh: The Missing Years, 1919-1941 (Berkeley: UC Press, 2003).

I am familiar with academic's works about Ho Chi Minh's life, such as "Ho Chi Minh: A Life" by William J. Duiker. One thing I noticed that many beliefs among Vietnamese community about his life are rarely backed up by academic sources. The "two Ho Chi Minhs" theory is created based on a "void period" in his life. The theory suggests that "Nguyen the Patriot" died in 1932. A different man, with the name "Ho Chi Minh", later claimed to be "Nguyen the Patriot" to take his reputation.

From 1918 to 1932, Ho Chi Minh was known under the name "Nguyen the Patriot". He came to the 1919 Paris Peace Conference to ask for independence for Indochina. He was one of the founders the French Communist Party and published many articles against French colonial regime. He traveled to the Soviet Union in 1924 to study, then traveled to China in 1927. He founded the Communist Party of Vietnam in Hong Kong in 1930.

There are few documents about his life from 1930 to 1940. According to William Duiker, Ho Chi Minh changed his identity and went to the mountainous region in Thailand, where he worked with a group of Vietnamese patriots. Occasionally, he also came back to the Soviet Union to attend meetings. To avoid French secret police, he made it hard to track him during these years. As a result, there are all sorts of myths created by many people, including himself, about what he was exactly doing in this period.

When WW2 started, he came back to Vietnam in 1941 and took a new name. "Ho Chi Minh" means "He Who Enlightens". From that point he led the communist movement in Vietnam.

Some Vietnamese believe Ho Chi Minh was in fact a Chinese spy, who assumed to be "Nguyen the Patriot". They provide some evidences such as he was fluent in Chinese and liked Chinese music. These kinds of arguments are hardly academic, so they should be considered as urban legend, rather than serious theory.

I am not familiar with this belief, and to the best of my knowledge there is no evidence to back it up. I can, however, see where it comes from (maybe), as there's a lot we don't know about Ho Chi Minh's life. Until quite recently, historians did not know what his birth name was, for instance. "Ho Chi Minh" is a pseudonym meaning "He Who Enlightens." As far as we know his birth name was Nguyen Sinh Cung. He had somewhere around 100+ pseudonyms, and scholars periodically discover new ones. He spent 30+ years in exile, and there's quite a lot that we don't know about what he did during that time. Incidentally, that's why he had so many pseudonyms; he lived under a variety of different names while abroad so as to be harder to track. Frankly, one of the main reasons for all of this mystery is that Ho consciously tried to remain extremely secretive about his life. After the First Indochina War, he insisted that he had not founded the Indochinese Communist Party under the name "Nguyen Ai Quoc" or "Nguyen the Patriot" even though he certainly had. Also, what sources do exist are scattered about several continents, since he traveled so much, while quite a lot of those sources located in Vietnam are still inaccessible to most scholars.