Its totally random, it is said that it can be drop by everything like mob, boss, chest and in any map.
My main is at around 16k kills and been unlucky in solo adventure or in full parties.
Ive witnessed 2 vanity helm drops... sadly not for me. Xray vizor in numa ruins at the beginnin of map. And 1 green mini alien head (forgot d name lol) in middle of lab.
Zbooo had 2 drops, guess hes the only 2 times lucky at moment.