In the Middle Ages and later there persisted a common belief that the Vandals were ancestors of Poles or Slavic peoples. That belief originated probably because of two facts: first, confusion of the Venedes with Vandals and secondly, because both Venedes and Vandals in ancient times lived in areas later settled by Poles. In 796 in the Annales Alamanici one can find an excerpt saying Pipinus ... perrexit in regionem Wandalorum, et ipsi Wandali venerunt obvium ("Pippen went to regions of Vandals and the Vandals came to meet him"). In Annales Sangallenses the same raid (however put in 795 is summarised in one short message Wandali conquisiti sunt ("Vandals were destroyed"). This means that early medieval writers gave the name of Vandals to Avars.
Very soon after that in chronicles the name "Vandal" started to mean "Slavs" (eg. in the same Annales Alamanici about a raid of Charlemagne in the country of the Polabian Slavs: perrexit in regionem Wandalorum). In 1056 Annales Augustani mentioned defeat of Germans with Slavic Lucics (?) as exercitus Saxonum a Wandalis trucidatur ("an army of Saxons is destroyed by Vandals"). In the chronicle of Adam of Bremen there is a longer sentence:
In 983-993 Gerhard of Augsburg in Miracula Sancti Oudalrici (about saint Udalric) called Mieszko I dux Wandalorum, Misico nomine.
Probably the first man who directly mentioned supposedly Vandalic roots of Poland was the Polish chronicler Wincenty Kadlubek in the 12th century, who wrote that Poles were once called Vandals, because they live next to the river Vandalus (Vistula), and that river received its name from the mythical queen Vanda who committed suicide by drowning in it. A similar story was told by the author of Wielkopolska chronicle from the 14th century, and then Dzierzwa from Krakow in the 14th century, who tried to give Slavic etymology to all known Vandalic names, like deriving Vanda from węda, that is fishing-rod.
In 12th century also Gerwazy from Tilbury, English writer in Otia imperialia wrote that citizens of Poland are called and are calling themselves Vandals. Similar thoughts gave German historian Albert Krantz (1450-1517) in Wandalia sive historia de Wandalorum vera origine, variis gentibus, crebris a aptria migrationibus, regnis item, etc where who consequently connected history of ancient Vandals and Slavs. The same was repeated by Falvio Blondi from Italy, and then Maciej Miechowita in Tractatus de duabus Sarmatiis... from 1517. Other arguments that Vandals were Polish ancestors were supplied by Marcin Bielski in 15th century. The first Polish historian to deny any connection to Vandals and to criticise that idea was Marcin Cromer, bishop of Warmia, author of De origine et rebus gestis Polonorum from 1555.
That idea is a perfect example how long absurd theories which were given by similarity of name can last.