Apostle to the Obotrites
Archbishop Adalbero II of Bremen dispatched him to the Obotrites (Obodrites, Abotrites, etc), a tribe of Wends living up around Holstein. As allies of Charlemagne against the Saxons, these western Slavs had a favored spot in the ninth century political order, but by 1147 they found themselves the target of a Crusade. Danes, Germans, and Poles ganged up on the Wends to forced them into political as well as religious submission. Vicelinus, who had been plugging away at converting these folks, found things both simpler and more complicated. It was simple in that suddenly, every Wend was Christian on penalty of death. It was more complicated in that "penalty of death" is seldom a way to win the local hearts and minds. Moreover, he as the new archbishop of Oldenburg, he was the Face of the Occupation. That's hardly the sort of position to endear oneself to the locals.
Even though Oldenburg was back in the hands of Christians, a Danish army came storming through and ruined it. "They clep us drunkards," said Hamlet. He might have added that they were right. Vicelinus remained bishop, but had to move the seat of his diocese to Bosau, which apparently the Danes left standing. A couple of strokes left him pretty impaired and he died not long after.
Vicelinus has also been called Vicelin, Vizelin, Wissel, Witzel, and Wizelin. His name has been associated with the personal name Wizo, also known in German as der Weise, 'the Wise'.