Please tell me please if there is Vietnamese community or community of people who speak Vietnamese in Wroclaw? Where can I find them?
There are probably about 5000 or so Vietnamese in Wrocław, and about 40,000 in the whole country. There are no reliable figures because many, if not most, are staying illegally. A very high percentage of them are involved in organized criminal activity, primarily smuggling and production of narcotics, cigarettes and counterfeit consumer goods, money laundering and other financial schemes, human trafficking, and fencing of stolen property and counterfeit consumer goods. Kidnapping, slavery and sexual exploitation of women and children are also regrettably common, as is extortion and racketeering, especial against other Vietnamese individuals and illegal immigrants.
You'll probably want to avoid this community until you become settled and can find and move within the small and closed circle of legitimate Vietnamese residents here. Doubly so for your wife. The worst enemy of Vietnamese people in Poland is other Vietnamese people. For every one that you might find that wants to genuinely help you, there are ten or more that want to exploit you. The fact that you have children makes you especially vulnerable.
As you are living in the Ukraine, you are probably already aware of a lot of this, as I imagine the situation is similar there.
What's kind of jobs that my wife could do if she move there?
Basically, legal employment will be all but impossible to find. Like winning the lottery. God help her if she has to rely on the Vietnamese community to find herself illegal work. She could end up getting killed so that the mafia can sell her heart and kidneys on the black market, a nasty practice called "human refrigerators". Or she can end up as a sex slave. Or worse.
Is Polish difficult to learn? How long do people need to study Polish to be able to communicate with Poles?
It's extremely difficult. It will take several years of very hard work to develop even a basic command of the language, and many years to develop a level of fluency that could be salable on the job market. By far most people who start learning Polish give up within a short time. Only a few ever become fluent.
If, on the other hand, you are near-native fluent in Russian, the time frame would be shorter, and the chances of success higher.