this was brought up during beta, along with countless other vampire bugs beth chose not to fix (as well as other general bugs that i saw during beta that never got fixed

)
it is not based on any slider nor anything you have done in your game, its a texture issue of some sort, a very rare few report not having the issue but if you do its probobly there to stay and there is not really anything you can do about it.
its very sad, vampires got really cool in this update, but they are to visually buggy for me to want to play one

and yes, apearence matters, ask anyone in the world anyware ever.