Andrew -- the one name alternative has some attraction, although you lose one piece of functionality that led to the creation of identifying names -- differentiating one person from another. His name is John. Yes, but which John? -- Oh, John, Patrick's son. John the Carpenter.
So it would seem to me that tribe does have some legitimacy. Gaiseric. Which Gaiseric -- oh, Gaiseric of the Vandals. Gaiseric the Vandal. Wikitree doesn't like multiple word LNABs, so "of the Vandals" doesn't work. Gaiseric Vandal would have some merit -- Vandal becomes an adjective there. The plural Vandals really doesn't work.
But those are English words. What language did they speak? The Vandal language whatever that was. Or in written form, Latin. My dictionary tells me the Latin form is Wandalica. Gaiseric Wandalica? Is that more precise or simply contrived? If he were an Ostrogoth, he would be Ostrogothorum. Or am I using the wrong dictionary? The problem of trying to use a language in which I'm not competent is that there's a high likelihood of error.
I'm moving toward recommending (1) the tribe (2) the single form, i.e. Vandal, not Vandals, and (3) the English version in lieu of the unknown Vandal form, versus the Latin. Vandal, Ostrogoth, Visigoth as LNABs.