If you want to get a quick answer where the Sclavi come from, all you have to do is close your eyes and say that name really fast over and over again.
BTW the same works for the Suavi.
🙂
Incidentally, if the Romans and Greeks inserted the “c” between the “S” and the “l” of “Sclavi because they found the “sl” sound hard to pronounce (as a lot of academics believe), wouldn’t it make sense that the “c” in Viscla was also inserted between the “s” and the “l”?  And if so aren’t we dealing here with an original sound that is the same as the Sclavi sound?
Note that Vistula had different spellings such as Viscla but also Vistla.  And ithe “t” there may have played a similar role as the “c”.  But what of “Sclabi”?  Well, the “b” and “v” sounds have changed.  Thus we have Suevi but also Suebi and also Suavi. Indeed we even have a Bisula designation for the Vistula in Jordanes’ Getica.
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According to Rozwadowski/GoÅ‚Ä…b *vis(t)la from PIE *ueis ‘various kinds of fluid’ with a slavic suffix *sla/*tla (like in maslo ‘butter’ from maź ‘smear’ and suffix *slo). There are some identical or similar synonyms in Dniepr basin as well as in Romania (Vasla), so the name seems to be proto-slavic indeed. So maybe not the same stem as in Sclavi (slovo/slava) but nonetheless it seems to point to an early occupation of Wisla basin (perhaps several centuries before the birth of Christ). Btw, thank you for some great articles; I always knew that Conan must have been among my venerated slavic ancestors 😉