For the question on Venice, this has been answered in better detail here by /u/Itsalrightwithme. Basically, Venice was territory of the Byzantine Empire. Charlemagne attempted to conquer it, but failed. Thus he properly recognized it as byzantine territory. Over time, Venice become more independent and wealthy, and subsequently harder to conquer.

For the question on Brittany, this question has also been answered in better detail here here by /u/kratistos. Basically, Brittany was at the very least de facto conquered by Charlemagne. Brittany was likely paying tribute, and was no threat at all to Charlemagne. Full on conquest was simply not a priority or even worth it at the time.

I don't know much about the Brettons, only that they were a rambunctious and independent people who were not easily align with the Frankish governing system, meaning that Charlemagne had no justification for war against them as far as I know, nor would he have been able to successfully govern them if he had contrived some reason to. The Venetian issue, on the other hand, is a matter which I know a whole lot about and can get into some detail.

When, in the eight century, the filo-Byzantine Italian ruling class defected to the Franks, Venice was one of the few Pro-Byzantine holdouts. This is both because social and political ties to Byzantium were stronger there than in other places, and the degree of military, political, and organization autonomy in Venice allowed the lagoon communities to defend their pro-Byzantine stance while other parts of Italy could not. Allow me to elaborate.

You probably know that coastal northeastern Italy was a Byzantine Military outpost up until Ravenna fell to the Lombards in the eight century. With Ravenna under siege in 751, no Consul was appointed to govern the lagoons that year. The people resolved to elect their own Dux, or leader (a title which would quickly be corrupted in the local Vulgate, to "Doge"). When Ravenna fell and what was left of the Byzantine army evacuated to Bari, this man (Teodato Orseolo) found himself the ruler of a de facto independent polity.

We all know the narrative; not long after the Lombard conquest of Ravenna, they turned their attention to Rome. Pope Adrian consequentially called to the Franks for aid, and Charlemagne spared no time to descend onto Italy and make quick work of the Lombards, asserting himself in Northern Italy and granting the Papacy a large temporal territory to govern. Charlemagne saw the lagoons and resolved to ignored them; in Nicepherus' 811 treaty of recognition with an elderly Charlemagne, Venice is explicitly recognized as a sort of self-governing Byzantine commonwealth. Why was Venice ignored? We find out when Pepin, Charlemagne's son and successor as King of Italy, later attempted to occupy the lagoon.

How effective the armed citizenry of the Lagoons could be against the landed-warrior-levy of the Franks is up for speculation: a decisive battle was never fought. Pulling up buys and markers so that only the most experienced navigators could sail in the shallow waters of the lagoon, the Venetians limited Pepin to the islands of Chioggia and Pellestrina in the southern lagoon and Grado, Jesolo and Eraclea to the north. But once the King arrived in the channel of Malamocco, he found sharp stakes driven into it, making easy crossing impossible. The Venetians pelted the Franks with arrows as they tried to wade the waters, and the Franks soon withdrew. They found they had to crowd onto the thin island of Pellestrina (which was obviously not nearly as built up as it is today) and stay out of sight of the shore: the Venetian boats circled them like hungry sharks, their arrows picking them off one at a time (and when the Franks were looking particularly short on supplies, the Venetians would even throw bread at them). Eventually, Pepin negotiated to have a large tribute paid to him and left.

So it certainly helped the early Republic that the communities of Malamocco, Eraclea and Grado were each separated by the mainland by lagoons up to 500km^2 of brackish water. Sure, the lagoons are only waist-deep in most places, but approaching an enemy over nearly two hundred square miles of open terrain while loudly splashing is possibly the worst way to start a battle. Plus, the lagoons are crisscrossed by deep channels, so traveling in a straight line is never feasible. Add in skilled boat-men who can expertly steer barges of archers, and you have a death trap on your hands.

Pepin's year-long siege had the effect of accelerating the urbanization of Venice, as people living in communities along the edges of the lagoon took refuge on the central Realtine Islands, where the modern city of Venice stands today. But in the eight and ninth centuries, the modern dense city of Venice was still a fray cry away: we're still looking at a series of disconnected islands with people living in thatched roof houses facing the water and orchards and vegetable gardens instead of paved streets out back. These islands were: Rio Alto (high banks, today near the eponymous bridge) Spinalunga (modern Giudecca), Lupario (the area around San Giacomo Dell'Orio) and Olivolo (modern Castello). The City of Venice wouldn't be recognizable until at least the 10th century (architecturally, until the 15th).

The lagoon would prove a valuable defense time and time again: In the 9^th century, when the Hungarian horde rampaged across northern Italy, it eventually set its sights on Venice. Doge Pietro Tribuno spared no time into preparing the populace for war. As Pepin had done the previous century, the Hungarians rapidly made their way up the island of Pellestrina, only to be halted at the channel of Malamocco. There they attempted to cross on small coracles, which were easily rammed by the Venetian ships storming down the straight at full sail. The victory was quick and compete.

To attack the city of Venice, only once option is viable: amphibious assault via ship. But it's not enough to be an expert navigator, you need to know the tides or you'll run aground: the a war in 1379 a Genoese fleet (expert navigators) got as far as the island of Chioggia inside the lagoon intending to lay siege to Venice, but the Venetians (whose fleet of war galleys had been defeated and were relying on commandeered civilian boats) were able to hem them aground on Chioggia and quickly surround them.

Thus, protection from invaders all but assured, the inhabitants of city of Venice could dedicate themselves entirely to maritime trade and enriching themselves and their republic, slowly under the centuries extending their economic influence into political matters, first establishing overseas colonies, and eventually becoming a powerful force on the Italian Manland. I can’t get into the details of every single expansion (pick up good old J. J. Norwich’s History of Venice for just that) but we you get into the details of Venetian politics and conflict management in this older answer.