I don't think they really share any similarity to the north other then the fact that the hill tribes are first men like the northmen. Also when I read aSoIaF I always thought of The Vale as Ireland/Wales. The culture and history have a lot of similarity. Furthermore in the GoT tv show many of the actors use their irish welsh accents wheras elsewhere in the series (Rickon, Joff) efforts have been made to supress them. This makes me feel that a lot of people interpreted the vale in this way. In purley gameplay tearms we don't want 90% of the factions (all but starkies and martell) using the same accent, using this accent adds depth to the gameplay.
You have to bear in mind that this is a collection of peoples interpretations of the text. Like in the TV series everything might not be exactly as you remember it, each section of the mod will be represented by that persons interpretations. that said if there was some sort of up cry against using irish/welsh voices for the vale I would ofc pull that decision but so far people seem to be in favor.