"Fryderyk" as a Polish spelling, and one page from the Academy of St. Gabriel (http://www.s-gabriel.org/names/walraven/polish/)
lists "Fryderyk, Frydrych, Frydryk, Friderich, Friedrich" as Polish names derived from the German, Frankish or French.
I can't really offer much more help than Patricia, sadly. All of the period texts I can find (so far) mentioning a Fryderyk as the modern, Polish form, are in Latin. So Fryderyk Jagiellon is Fridericus Jagellonidis, for example:
And Fryderyk Getkant (Lithuanised as Frydrichas Gedkantas) is Fridericus Getkant
It looks like the modern Lithuanian form is Frydrichas. The only thing remotely like Frydrichas, that I can find, is in a Czech manuscript from 1619:
"Nasse Frydricha, z Boj Milosti Krále Ceskeho, Falckraběte při Reynu, a Kurffirssta, rc. Otewřené Rosepsánj"
This is apparently Frederick V, Elector Palatine, King of Bohemia. If Wikipedia is accurate, then his name today is usually spelled "Fridrich".
> "Venslauskas" seems to be a plausible Lithuanian surname, and maybe "Vinslauskas" too. However, I have no idea whether those surnames existed before 1600. Searches on these names tend to produce links to LinkedIn, Facebook, Ancestry.com and site of that ilk. Again, it would really help to know where the submitter got this name in the first place.
I asked this on Oscar, but it sounds almost like a corrupted form of a patronymic form of Wenceslaus? The link Patricia gave says that the Polish form is Wacław, which we can find in Latin-language Polish documents, such as "Venceslao Lescinio"/"Wacław Leszczyński"
Looking at "The Latinization of names in the Middle Ages" (http://nicolaa5.tripod.com/articles/latin.html)
a Polish name in Latin could be Fridericus Venceslaus? I honestly am not sure how correct that is. But it seems close to what the submitter wants...
Not sure how useful any of that is, I'm not very good at Polish names!