Wisdom damage is the lamia's standard natural attack. She doesn't need Improved Unarmed Strike since she already has an attack which causes her to be considered armed.
A lamia's normal attack doesn't cause hit point damage. Thus I would regard a damaging blow as a different form of attack that prevents the lamia's Wisdom-damaging touch from working. I'm by no means saying this is a general rule for touch-based effects, though.
You have probably considered this, but most lamias don't qualify for monk as they are chaotic.