Oh, my apologies if it hasn't been clear. It's not a plural, but an adjective. Anything from Vicenna is Viceni, just as anything from Areta is Aretan.

Not sure what the noun form would be. Vicennan I guess?

In RP settings, nationalities that don't follow the usual custom of "Thatplace-ian" are difficult to catch on. When I used to roleplay in Ultima Online, there was a guild of RPers from a city called Yew. Half of us called them Yewish, and the other half Yewians.

Both descriptors are pretty cringeworthy in their own right though.