Technically, Vayne is a Demacian vigilante, yes, so there's a person IN Demacia who shoots ranged attacks I guess. But Demacia lacks a truly Demacian ADC. I want one. Demacia has bruisers, APs, supports, even a semi assassin as of Fiora. Why no ranged ADC? Demacia needs an ADC.
Let's look at it on the whole. Jarvan can solo top/jungle, and trade with xin depending. Lux or Galio can mid or support, and switch with each other accordingly. Who's the other bottom laner? I guess Garen or Fiora could try it, but really, that's asking for trouble. If things get sticky, they'll be kited for days.
TL;DR Demacia does not have an ADC and it needs one to have the full five man team comp. And come on, there are houses in the Demacian royal family that haven't even released champions yet. An entire world of viability is there. USE IT RIOT. PLEASE.