Who were the Veneti? This is not an easy question to answer.
Baltic Veneti
The Vistula Veneti were called Wends, by German-speaking people. But another theory is that Veneti is a diminutization of venus, as per Latin origins. And for further confusion, Slavs living near Germanic settlements were also called Wends.
The Vistula Veneti hailed from Eastern Poland.
Adriatic Veneti
The Adriatic Veneti lived in a region that we now identify as "the area around Venice". Indeed, this area is known by the name Veneto, or Venetia.
Strabo, the Greek historian, conjectures that the Adriatic Veneti are related to the Veneti of Brittany.
Breton Veneti
The Veneti of the Breton Peninsula, were a tribe of Gauls which were defeated by Julius Brunus Albinus, in a naval battle in 56 BC, as part of Julius Caesar's campaign to pacify Gaul.
Part of the drama here was that the Veneti were experienced in operating on the ocean, whereas the Romans would have only ever known the waters of the Mediterranean Sea. In particular, ships sailed by the Veneti were faster than the Roman ships.