Not having watched all the series, it suggests to me that Poland might have had an under the table agreement with Vietnam to take in people who wanted to leave; maybe unemployment (unoffically, of course, since in that era Communist countries stuck to the story that they didn't have any) was rising as Vietnam's reforms took place, but Poland needed workers because they had managed to solve their economic issues. Likely, the official text of any agreement was more along the lines of soft soap about "fraternal cooperation within the framework of COMECON to permit guest workers, etc."
As to how that came about - I've said bits and pieces previously, but a fairly coherent version probably goes thus:
Jaruzelski, after the 1983 bombings, manages to effectively quell dissent and prod the population into accepting the hardships they previously militated against in order to radically boost economic output by keeping them working all-out in the factories and farms.
The Soviet Union kicks oil and gas into the pot, probably almost for free, in order to subsidize Poland's short-term economic expansion.
By 1986, changes in the Soviet leadership and Jaruzelski turning over control to PZPR civilian leaders allows the PZPR to trumpet relaxations of economic controls as the "next logical step in the evolution of socialism, our reward for years of struggle after the terrible bombings, &c."
Poles, just happy that they're getting the 'bread' part of 'bread and circuses', begin starting up private enterprises both on and off the books (recall that the Poland we see in this series has a thriving underground bar and club scene that caters to some pretty, ehm, wild tastes in some respects) and this, like China, initiates the virtuous cycle of continued economic growth with higher-quality goods and services. Thus, the need for Vietnamese workers and tolerance of organized crime factions getting in on the import-export scene.