Wikipedia map of attested and/or supposed Vandal migrations - I have the impression it also includes the Suebi (light yellow). The Vandal homeland during Roman times is commonly assumed to be Silesia and the middle Vistula, but I don't know which sources this assumption is based upon.
Migrating Germanic tribes appear to often have assimilated rather than displaced previous inhabitants, as is, e.g., demonstrated by the survival of certain Celtic habits is SE Germany.
http://en.wikipedia.org/wiki/Walpurgis_Night
OTOH, there is also evidence of some German tribal names, e.g. Rugii->Rujani having been taken over by Slavs in the 8-10th century. After having read the study posted by Zanipolo, I have little doubt that the Vandal
elite spoke an East Germanic language closely related to Gothic. But the population may well have included other linguistic groups, including proto-Slavs.
After arrival of the Huns, a substantial part of the Vandals, together with Suebi and some Alans, started their migration through Roman lands. The study posted by Zanipolo attests some Alan names found on tombstones in the North African Vandal empire, so Vandals seem to have had a tradition of leading multi-linguistic entities. While many Vandals migrated, others appear to have staid in Silesia. They may well have become slavicised, be it because the original population already included proto-Slavs, or due to Slavs immigrating into Vandal lands. As such, we possibly talk about a Germano-Slavic conglomerate that initially spoke East Germanic, but in Silesia later switched to Slav language (just as the North African Vandals became Romanised).
Asides from that, there has been a lot of confusion stemming from the fact that some medieval scholars equalised Wends (West Slavs) and Vandals. The Wendish Quarter of the Hanseatic League (Hanseatic Cities on the Baltic Sea, presided by Lübeck), e.g. was in the 14th century Latinised into
vandalicae urbes, even though none of those cities (Rostock, Stralsund, Visby, Stockholm, Kaliningrad, Riga, Tallinn, etc.) was even close to the territory inhabited by historic Vandals. The same applies to the
Ducatus Vandaliae (Herogtum Wenden = Mecklenburg-Güstrow, south of Rostock). The root of this equalisation is commonly seen in the founding legend of the Ducal House of Mecklenburg, the Obotrite family, who claim Vandal descent. [
Note that this line of argument would have made the Netherlands a Vandal territory, since Queen Juliana of the Netherlands was a born Mecklenburg (Obotrite).] That founding legend, in turn, has inspired various German and Polish scholars of the 14th to 19th century to "prove" that Wends were always German, or Silesia was always polish, respectively.