I am glad to answer this question, I believe I know Vietnam and Vietnamese in general more than most of Koreans do - I’ve stayed in Ho Chi Minh where my father currently lives in.
Therefore, I might have somewhat biased opinions (towards positives) on this issue, but I will try to be objective as much as possible.
1. Innocent and kind people:
Koreans think that Vietnamese people are mostly hospitable, and helpful. I felt exactly the same when I visited Vietnam, people smile more often, and don’t hesitate to help other people (especially to foreigners).
It is a bit different for most Koreans, people are rather be cold-minded, goal-oriented, very much individualistic (self-centered), busy, and competitive each other. However, it’s not because Korean culture or Koreans themselves are like that, but because many of Koreans live in the cities - I live in Zürich in Switzerland, the stereotypes towards Zürich people are exactly same in this society as well.
2. Great foods (ever!):
The Vietnamese traditional “Pho” is liked by all the Koreans, I would say. Vietnamese foods are very similar to Korean dishes, we love noodle soups, sea foods, and vegetables. Even though I like all the Asian foods in general, I can’t imagine eating Chinese, Singaporean, Thai, Malaysian, and Japanese foods for every meals, but if it is Vietnamese foods, there wouldn’t be any problems.
We know that Vietnamese are very diligent people as much as Koreans are. Both societies were heavily influenced by “Confucianism culture” in the society in general. Accordingly, diligence is one of the most important virtues, both people tend to pursue it.
4. One of the most prospective societies:
There is no doubt on this. I would say all the Koreans will agree that Vietnam is the most promising country among developing societies. Due to this expectation, there are lots of “Korean money” invested into Vietnam, not only private, also many of public sectors.
Many people wish we could share our know-hows on shaping, and developing the societies to Vietnam, and also wish Vietnam could become a fully developed society in the near future as we did. It might be partly due to the a sense of guilty from Vietnam war (we all know what Korean soldiers did to many of Vietnamese people…), and also a strong sense of kinship - we were also totally devastated by Korean war, and it took lots of efforts to build up society again.
Even though Vietnam is truly vibrant, and promising society, there are some negative characteristics as most of the people from a third world have (which I won’t mention here). However, those stereotypes exist not only in Korea. At least, those stereotypes towards Vietnamese (and other South East Asian countries…) exist in the two other countries I’ve ever lived as well - Australia and Switzerland.
Also, Koreans normally don’t really care about those seriously in a daily life. As long as you respect other people, and behave in a well-mannered way, you will be respected equally. “Politeness” is very important virtue in Korea, people would not disregard you simply because of your nationality, ethnicity, or any kind of backgrounds.
Despite of a negative prejudice towards how Koreans think of Vietnamese here in this page, I would say Koreans have pretty much good impressions and a sense of kinship towards Vietnam in general.