I just sent an email. Couldn't find the original post here but did see Sara saying they improved the knees and hips on Teegra and that they didn't have similar problems with Darkwolf (I haven't had issues with him myself).
Of course I'm not assuming the Vampirella I have is also going to break if I move her knees. I have ... shoot, one of the other lines? Some other version of Vampi though in her black costume that doesn't have quite so brittle parts (though she does not have quite so bodacious a booty, if I'm being totally blunt).