9 countries have legalized prostitution: Germany, Netherlands, Austria, Switzerland, Greece, Hungary, Latvia, and Turkey.[0]
Human trafficking, in order of severity, is most linked to: Netherlands, UK, Romania, France, Germany, Bulgaria, Italy, Poland, Hungry, Portugal.
From this list alone (these tend to follow power laws) there's a higher correlation with being a Bloc State than legalized prostitution. Unsurprising to see UK and Germany here considering they are not only the most wealthy countries, but also the most populous. Netherlands... well I got no answer there but this is just a quick look.
Just given this quick look, I'd be shocked to learn that legalization of prostitution was a major causal factor in human trafficking.
[0] https://en.wikipedia.org/wiki/Prostitution_in_Europe
[1] https://www.statista.com/chart/4947/the-eus-hotspots-for-peo...
This isn't the same as saying that prostitution is legal because the transaction is still illegal. It also doesn't resolve the issue of the correlation, which is the main point.
And, no, women who move around a lot and live on-site isn't proof of trafficking--because that's also a way to make it much harder for the cops to investigate and thus a sensible safety strategy for them. I'm sure trafficking exists but I don't believe the claimed numbers at all.