There's very little that can be said for certain about Tacitus account. For many of these groups, the mention there the only one that exists until centuries later, or at all. Some are almost undoubtedly fiction or legend, such as the matriarchal Sitones, who are not a good match in name or culture for any known group in Scandinavia or the Baltic area.

On the other hand there are groups with names that are more settled-upon, such as the identification of the 'Suiones' for the Svear or Swedes-proper, the neighboring 'Aestii' for the Estonians, east across the 'Suebian sea') (Baltic) from them, and the 'Fenni' which are the Finns and/or Sami people. And it's in this context the 'Veneti' turn up, and the term is usually taken as referring to the Wends, which in how it is used in the first written sources of the region, centuries later, is a term for some proto-Slavic people around the Baltic. But I'll get back to that bit.

First one should perhaps address how Tacitus knows any of this. He himself had, after all, not visited the area. But it's fairly clear that he is primarily relaying things he's heard directly or indirectly from Germanic sources. (and of course his own conjecture based off it) This would align well historically with the fact that by this point in history, significant numbers of people from all over the Germanic world, up to and including Scandinavia, were journeying to Rome and (among other things) serving in its armies. The cultural impact was immense and lead to that era (1-400 CE) being dubbed the 'Roman Iron Age' in the archaeology of Scandinavia and north Germany, due to the sudden preponderance of Roman artifacts and roman influences in the archaeological record. (in a likely imitation of the Romans, the Runic writing system was also created around this time; although which specific Italic or possibly Greek alphabet was the inspiration is a matter of debate)

So that explains how Tacitus would get that information, but it's also seen in the information itself; The location of the Aestii is described relative the Swedes, and their language is described in contrast to theirs as being different and more like the 'British'. Which it surely wasn't (as the Baltic region is completely devoid of Celtic names and language evidence)

Then we have the Fenni. (also mentioned contemporaneously by Ptolemy as φίννοι - fennoi) Which in its Old Norse form finnr referred to Sami/Laplanders but also in cases to those called Finns today (Suomalaiset), which is a different people and much more closely related to Estonians culturally and linguistically. The exact etymology is unknown but has been speculated to be a cognate of 'find', referring to a hunter-gatherer lifestyle. In any case the word is of Germanic origin and an exonym; it is not what the Sami nor Finns called themselves, it's what their Germanic neighbors called them.

So getting back to the Wends here; it is the same story; that term was likely used as a catch-all term for Slavic peoples in the Baltic neighborhood. When it comes, for instance, to the 'Wendish crusade' of the Danes in the early 12th century against the Wends of Rügen. (who were identified as a separate group, the Rugii by Tacitus) The term is commonly taken to refer primarily to them and related West-Slavic groups on the coast from Lithuania to Germany.

On the other hand you have the fact that the name of Russia in Finnish, Estonian and close relatives are cognates of 'Wend', namely 'Venäjä' and 'Venemaa'; indicating the term was being used for the future-Russians as well. Then you have to add even more uncertainty, since this is all many centuries after Tacitus!

So in summary: It's already a guess, although the most would say a decent one, that Tacitus' Venedi was a Latinization of the Wends' name, Proto-Germanic winidaz (minus the -az nominative ending and adding the -i for a Latin plural) and used in the same or similar way as it would be later; as a generic term for Slavic people in the Baltic region. So it can't be equated with any specific present-day nationality. There was never any protest that, after the aforementioned crusade, the Danish king (and later, the Swedish one as well) declared himself 'king of the Wends'. A title they only abandoned in 1972-3 after apparently facing up to the fact that that people hadn't existed in any meaningful sense since the Middle Ages.

If it's not obvious, Tacitus' attempt to classify he groups into Germanic and Sarmatian peoples is simply not a correct idea, and as that very quote shows; is based mainly on what his perception of their lifestyle is. It bears repeating also that there's so many fantastical elements in Tacitus story that it shouldn't be taken too seriously.

Also, this map off Wikipedia (which seems to take Tacitus quite literally) is hardly reliable either; seeing as there's no explanation of how the decision was made to put these names where; including groups that never existed as far as we know, like the Sitones. (not obvious, either why the Aestii would be in Latvia rather than Estonia, the Rugii not in Rügen, the Angelii not quite in Anglia and so forth)