[tremon]

It's an open question whether the Veneti themselves were ethnically Slavic though. We know of the earlier Gallic Veneti, which were a Celtic seafaring tribe that settled multiple locations along the European coastline. The Italian region Veneto and the city Venice can also trace their names to an ancient group called "Veneti", and they seem to have spoken a mix of Italic and Celtic. In contrast, the Slavic language seems to have originated much further east.

https://en.wikipedia.org/wiki/Venetic_language :

A 2012 study has suggested that Venetic was a relatively conservative language significantly similar to Celtic, on the basis of morphology, while it occupied an intermediate position between Celtic and Italic, on the basis of phonology

https://en.wikipedia.org/wiki/History_of_Proto-Slavic#Pre-Sl... :

The most archaic Slavic hydronyms are found here, along the middle Dnieper, Pripet and upper Dniester rivers [..] inherited Common Slavic vocabulary [includes] well-developed terminology for inland bodies of water (lakes, river, swamps) and kinds of forest (deciduous and coniferous), for the trees, plants, animals and birds indigenous to the temperate forest zone, and for the fish native to its waters

The fact that the Slavic language seems to have developed inland but the Veneti mostly settled coastal areas puts a big question mark under the Veneti == Slavs link.