Your average Vietnamese restaurant in Poland is less than desirable, consistently cheap and often in a shack like building. Meals tend to consist of rice or rice noodles with veal, beef, chicken, pork, duck and seafood in a choice of sauces which all tend to taste rather similar. The meat can often be chewy and the choice of starters is usually just as appetising.
An abundance of cabbage usually comes with the meals which can be used to disguise the taste of the meals or simply cheer up the locals. Vietnamese food largely pulls in a crowd of workmen, drunk people on the weekend or hungover people on the weekend.
The amounts succesfully satisfy the hungover and the grease soaks up the beer but to the less blurry eyed there are few of these restaurants that appeal nd to the foreigner who may have tasted better Chinese fare in their home country, will ultimately be a disappointment.
A large amount of Vietnamese headed to Poland in the late 80s and early 90s, a fair proportion of them sell trinkets some have done well over the years, particularly in the clothes market.
Another option, particularly for the legal residents is to set up a restaurant - judging the food, I can only think that their profession isn't in the kitchen as it is now and that could be one of the reasons for the lack of quality, Poles don't tend to use a great deal of spice in their cooking and this could also be one of the reasons, otherwise it could just be that such a huge amount of meat, veg and sauce is hard to knock up for such a cheap price without the meat having to come from suspect supplies.
Either way, unless you want Pol Pot running riot in your stomach, I'd tread vary carefully and not just step in to any old Asian take away in Poland if you want your meal to be enjoyable.