I didn't hear a Jersey accent when he did the voice of the Iron Giant. If there's a big concern with Vin saying 'wooder' (water), then hire a dialect coach. I actually think his deep voice would work well with Vision. Did Christian Bale sound Welsh when he was Bruce Wayne?
Others are basing their opinions for Giantman because of him standing in front the comic with GM being so prominent, but I was drawn to his comment about 'tunnel vision'.