I know this is probably a dead horse by now, but Vader's "wobbly" helmet has always bugged the crap out of me too. So, I still have my fingers crossed, hoping it will be fixed. I think that this quote provides an adequate justification for my feelings as well:
Originally posted by: auximenies
Besides, the flimsy looking helmet doesn't jibe with the sound effect of his helmet being secured to his scarred head. SHHHUUUMP! That certainly sounds like a secure fit to me.