Extract who has logged in/out of vSphere

Hi,

I've been asked to produce a report of the people who have logged into the vSphere client for the past 6 months. Looked on the web but i wasnt able to find a great deal of information. Does anyone know if this is possible. I assume the database stores this information but im unsure how to extract it from there.

Running Virtual Centre 5.1 U1a

Virtual Database is SQL

Thanks