User blog comment:AYET/About chapter 691 - Vergo, the pirate?/@comment-1595565-20121206042428

He works for Doflamingo, who is a pirate. Therefore, Vergo is also a pirate. They called him a pirate because for the longest time they thought he was a vice admiral, who are normally addressed by title, ie Vice Admiral Vergo. So, since he turned out to be a pirate, they call him pirate Vergo. You're really overthinking this.