The article says prostitution is illegal in Thailand but is not really true. The 1996 Prevention and Suppression of Prostitution Act only makes prostitution illegal in certain circumstances.
Soliciting in public so as to cause a nuisance is prohibited.
Operating or managing a prostitution business is illegal.
Profiting from the prostitution of others (e.g., pimping, brothel-keeping) is a criminal offense.
The VN hookers may well be bothering people in the street which is clearly illegal but It is a hassle for cops to get good enough for that. Passers by will not give evidence. So the cops have to do it. In the past to prove the girls (East Europeans in those days) were really soliciting for sexual services, cops actually had sex with them, arested the girls when they had finished and knotted up the used condoms to present as evidence. (I think there was a queue of cops at Lumpini police station volunteering for this dangerous mission confronting violent criminals.) There was a big outcry from Thai women's groups arguing the cops had created the crimes themselves as agents provocateurs. There was also an outcry from the wives of the arresting officers who were named in the media. Ha ha. So the cops were forced to abandon these fun crime busting missions.
Nowadays, if they take action against foreign hookers at all, it is most likely to be for visa violations.
The cops don't like to admit prostitution is not really illegal in any meaningful sense the same way as they pretend smoking weed in public is specifically illegal. It is not but smoking anything in public is illegal under the Public Health Act, if it causes a nuisance and someone complains. So it is actually a fairly similar concept of illegality that makes it hard to get convictions. So cops don't usually try in either case.