China and Vietnam share a long history and culture. They are also right next to each other and have the same government type. Vietnam has openly showed their desire to work with China at the cost of the US. They sent some top diplomats to China right after the US diplomats came to visit them. That is pretty clear to me which country they prefer more.
Investing in Vietnam is basically the same thing as investing in China in the past. It will end the same way, with a strengthened opponent in Asia.