Thoughts of lust and thoughts of vanity can be conjured up to defeat each other. But vanity is particularly difficult to defeat. If you have defeated the other seven thoughts, you are likely to have thoughts of vanity.
Sorabji, following Evagrius, argues that vanity occupies a uniquely resistant position among the eight bad thoughts because it is self-reinforcing and persists even after all other passions are conquered.
, Emotion and Peace of Mind: From Stoic Agitation to Christian Temptation, 2000thesis