This isn't my field, but from what I'm reading it seems the difference in intensity of debate is related to the issue of Gothic ethnogensis being tied up in ideas of Swedish national identity (Neville 213-4, 234), whereas the ethnogenesis of the Vandals not only has little evidence but is not tied up in any comparable nationalistic debate. It seems a Swedish historian is probably more likely to argue the Goths are from Sweden or somewhere nearby (Strid 2010) whereas a mainland-European historian might be more likely to argue the Goths were from Central Europe (Kortlandt 2001). This might also be a gross oversimplification of the debate, but you can look at the debate over whether Alexander the Great was Macedonian or could also be considered a "Greek" for an example of how nationalism can complicate historiographical debates.

The evidence for the origins of either is pretty shaky due to the Goths and Vandals both being "prehistoric" in that they did not keep written records before their first contacts with the Roman Empire (Bennett 20). The oral histories of these people as transmitted by important sources such Jordanes' Getica may very well have been fabricated in the first place by the Goths themselves (Bennett 23). Even if we assume that the Goths were a clearly identifiable gens that can be geographically tracked to their ethnogenesis, which is a shaky proposition anyway, the evidence does not exist to give us a precise answer, and the only new evidence we are likely to come by in the form of archaeology may be of no help in determining evidence on ethnicity (Liebeschuetz 210-4).

As for the Vandals, we have even less evidence: for instance, aside from their lack of writing we also have no origin myth (Turner 177-8). The well-informed appear to suggest that the acts of migrations and combinations of peoples that resulted in the formation of the Vandals means that they while they formed into a group recognisable as the Vandals in Central Europe and continued to solidify their identity as they moved West, but more than that does not seem possible to determine (Turner 176-8).

I've never looked into this topic of Gothic or Vandal ethnogenesis before, and I have to say it appears to be quite a depressingly inconclusive field. Perhaps we will have to make do with the general assertion that the Goths and Vandals were both of "Germanic" origin (Wells 596), whatever that means. Due to limitations of evidence, there are unfortunately questions in ancient history that we can't always answer, and at a glance, this to me seems to be one of them. Jordanes tells us the Goths were from Scandinavia, but the debate over the accuracy of that assertion can probably go in circles forever. The origin of the Vandals is even more shadowy, and considering the fluidity of racial identities in this period of confederations, migrations and Germanic Romans, perhaps it is enough to say that the Vandals originated somewhere around Central Europe, formed from various Germanic peoples of various origins.

Sources:

Bennett, Medieval Europe: A Short History, 2011.

Kortlandt, The Origin of the Goths, 2001.

Liebeschuetz, Making a Gothic History: Does the Getica of Jordanes Preserve Genuinely Gothic Traditions?, 2011.

Neville, Gothicism and Early Modern Historical Ethnography, 2009.

Strid, The Origin of the Goths from a Topolinguistic Perspective: A Short Proposal, 2010.

Turner, Konflikt und Anpassung: Studien zu Migration und Ethnogenese der Vandalen (review), 2009.