Vendetta (a.k.a. feud) is when you fight for your right personally, instead of having it given to you by the state's monopoly on violence. This was a common practice in the medieval age, which is why we call it Feudalism: Landlords had to guarantee security to their subordinates in exchange for their labor, and they had to militarily implement that security guarantee by feuding for them in their place, in case they were attacked or otherwise unfairly treated.
The Ewiger Landfriede ("everlasting Landfriede", variously translated as "Perpetual Peace", "Eternal Peace", "Perpetual Public Peace") of 1495, passed by Maximilian I, German king and emperor of the Holy Roman Empire, was the definitive and everlasting ban on the medieval right of vendetta (Fehderecht). In fact, despite being officially outlawed, feuds continued in the territory of the empire until well into the 16th century.
Claims were henceforth no longer to be decided in battle, but confirmed through legal process. The imperial act was passed on 7 August 1495 at the Diet of Worms. In theory, at least, the use of violence to resolve disputes was replaced by settlements in the courts of the empire and its territories, even if the establishment of this principle took several further generations. In a modern sense, the Ewiger Landfriede formally gave the monopoly on violence to the state or the public sector.
[...] The state acknowledges the right of individuals to ensure their own rights by force only in very limited circumstances (e.g., in self-defence). The monopoly of the state over the use of force has its root in the medieval state peace movement which prevailed in the 15th century.