Yeah, its probably not the best design but its got to be hard to make it work perfectly for everybody. Some bikes dont do it, some are bad some not so. I havent heard of anyone having trouble with vibration in clean air though. Before i fixed mine, i had to ride standing up if i was going more than 40kph, that or take the trail bike.
Also, its not exclusive to Arai, other peaked helmets have the same trouble sometimes as well. Shoei hornets included (even with their 3 mounting points on the peak)