Slavs settled here in the 6th century during the the period of Slavicization. It had to do with the domination of the Avar Khaganate to the south. The same migrations also saw the settlement of Polans, Morvanians and Czechs, to name a few. The slavs on the Baltic sea that you are asking about were called Pomeranians, (from Slavic Po Mer, "by the sea"). They were engaged in a long conflict with the Piast dukes of Poland, who had settled further south. By the 11th century, the German Drang Nach Osten "push for the east", brought the Saxons into the mix. It was the Polish who defeated and converted them to Christianity in 1121. The Saxon migrations were pretty successful, but in the Baltic regions their progress was hijacked by the Teutonic Order. The Teutonic Order was invited by Poland to help deal with Prussian agression. The Prussian Crusades were about 100 years after the Pomeranians had been converted to Christianity. The reason more Pomeranians didn't revolt is because of the Germanization that had taken place. The Pomeranians on the Vistula would have been a powerful tribe to due the strategic commercial interests of the Vistula. They were also adjacent to the Prussians. It is understable why, as outlined in your map, Pomeranians on the Vistula would be allies of the pagan Prussians. River deltas were often a place that hostile tribes would control for their natural defenses, similar to islands.
Edit: I see that you are looking for concrete proof that Slavs inhabited the region. I don't see much reason to question that they did. Explanations and maps seem to universally show them at the mouth of the Vistula. There should be more to go by than archaeology in the Middle Ages.