Since you don't seem to have any predisposition towards any of these languages, difficulty and practicality are probably the most important factors you should consider. I'm completely ignorant about Vietnamese, so I won't comment on that. As far as difficulty is concerned though, Chinese and Japanese are the hardest, with Chinese marginally being the more difficult of the two.
One of the hardest aspects of Chinese is the writing system (obviously). I can't say how difficult it is to master pronounciation, but it seems like an intimidating challenge. Despite these challenges, I have been told the grammar is similar to English, or at least far more similar than Japanese or Korean, which may make it easier to pick up than the others. It's also easier for Chinese to guess the meaning of things written in Japanese than the other way around.
Japanese uses both Chinese characters and two syllabaries, but thankfully you don't need to learn as many total characters as Chinese for mastery of the language. The difficult aspect is the grammar, which is completely unlike English. As a beginner, this, more than pronounciation or vocabulary, is what will limit what you can say.
Korean has a simple writing system that can be learned in just a few days, saving you the headaches of the Chinese and Japanese writing systems. The grammar is very similar to Japanese, which is what would make it difficult, but otherwise it is the easiest.
Economically, Chinese is your best bet. It is spoken by more people than any other language, and the business opportunities should be fantastic in the future as China modernizes. It is the only country with a larger GDP than the United States, so even now I would wager you would have better luck making money with Chinese than any other Asian language. The economy is growing at a good clip too, which is only making Chinese more and more attractive for business.
Japan has a large economy, but there is virtually no growth. Also, as Hournda mentioned, the population will contract in the future as the birthrate is extremely low. When the top-heavy population starts to retire, the shit is really going to hit the fan, and it is hard to predict how this will affect the relevancy of learning Japanese. IIRC the Japanese government already wants to see more (tax paying) foreigners working in Japan to prevent impending disaster, so it may work in your favor, or it may not.
Korea has the smallest economy of the three, but it's still larger than that of Mexico, Canada, or Austrailia, and it is growing at a steady rate. It can be useful if you are interested in technology/electronics. I think it is worthwhile to pay attention to this country and see how its economy develops.
How long do you need to study a foreign language for your major? It will take a long time for you to become good enough at CJK to apply it to a job, so if you are only taking it for one or two years, it's probably best to just take the easiest one instead of the most useful.