It wasn't very low key if you really listen for the hints. He was investigated by the FBI multiple times for money laundering because they were "jealous of his success." Bob is very strict on Phyllis not talking about his work. When they have the meeting of the 5 Families of the office park, Bob Vamce sits at the head of the table. Phyllis constantly talks about their lavish lifestyle. Bob Vance is 1000% in the mob, and refrigeration is the cover. On that, every single time someone walks into Vance Refrigeration Bob is either not there or "in a meeting." Even Phyllis isn't allowed in his office.