Thanks. Didnt know if it was... how should i say... "proper forum etiquette" to make up something that was never mentioned in a game.
I do it all the time in my RPs. Usually I just give a brief yet descriptive outline, of what the place looks like. Then I let the players, use their own imaginations to fill in the blanks where they like. Maybe find some pictures of Hawaii, and put them as places your RP will center around? Of course, they'd be places you though could fit and what not. And maybe for factions (if you want factions) just base them off an already formed and known Fallout faction, like NCR Rangers for example, or something like that?
But I suggest, that since Hawaii is such an isolated and small place. (some areas that is) then I would probably keep the RP to a tribal or maybe rugged Merc or some kinda formed outfit type vibe. Do some research (not much, but enough to get by if you like) on Hawaii, and it's culture/habitants (creatures and people) and that should give you an idea of where you could take your RP.