This is from Philippe Dollinger, “The German Hansa” 1970. Dollinger is basically the top 20th Century expert on the Hanseatic League. This first part is kind of the origin-story of the Victual Brothers:
“In 1389, when a rebellion of the Swedish nobility drove Albert [of Mecklenburg] from his throne and put him in prison, she [Queen Margaret of Denamrk] became also queen of Sweden. Stockholm, however, remained loyal to Albert, thanks to the support of the German population of the town.
In their almost desperate situation the Mecklenburgers decided to wage the war of piracy with added vigour. In a sensational proclamation they promised to open their ports ‘to all those who at their own risk would got to sea to harm the kingdom of Denmark‘. The appeal was highly successful. Knights, townsfolk, peasants and gallows-birds hastened to enlist under the leadership of the Mecklenburg nobility. Rostock and Wismar became pirate bases, where ships were armedand equipped, raids planned and booty safely stored and divided up. Piracy soon made navigation almost impossible in the Baltic, for the corsairs did not confine their attacks to Danish ships. The Hansa not only suffered material loss in the war, but was once again torn by an internal crisis, when two of the Wendish towns, through self-interest and loyalty to their overlord, dissociated themselves from the common cause.
It was in this period that pirates were first called Vitalienbrüder, a name which has remained associated with them. It is of French origin. At the beginning of the Hundred Years War, the vitalleurs were the soldiers responsible for supplying the armies, which they eventually did by brigandage pure and simple. At seat the name was given to ships carrying supplies to fleets and ports, and finally it reached the Baltic, where it became synonymous with pirate.
Piracy enabled the Mecklenburgers to win some resounding successes. They were able to succour the garrison of Stockholm on several occasions, to carry out devastating raids along the Danish and Norwegian coasts, and to inflict serious losses on the Danish fleet. In 1391 the Mecklenburgers captured Bornholm, Visby, which provided them with an excellent base for their operations, Abo, Viborg, and various strongholds in Finland. Two years later Bergen was sacked, and its inhabitants took an oath of loyalty to Albert of Sweden. In the following year Malmö too was pillaged.”