It seems like the practice could well share inheritance with ancient Greece, but I'd be surprised if the motivating mythology would be quite the same. Wonder what the story was in ancient Poland..
I suspect this one may have more to do with countering external threats.
https://scifi.stackexchange.com/a/38468
This would imply that helping the soul cross the river doesn't prevent "vampirification".
Some 18th century people didn't like it because they thought it was too colloquial but its been a part of English for a fairly long time, roughly 10 times as long as the "Internet age".
I think it's quite common to see front lines and new borders form around ethnic or cultural borders, instead of existing / established country borders. Or that there's clear cultural differences on old borders, like for example where the Roman empire's borders were, or the borders between England, Wales and Scotland in the UK.
There has been a lot of history since the days Roman empire.