Vice Officers are responsible for curbing the most blatant excesses that a crew might indulge in, specifically hard drugs, excessive alcohol, or sex trafficking (as opposed to sex without money directly involved.) Instead of Medbay and Research Sector access like regular Security Officers, they get access to the Bar, Kitchen, and Hydroponics. They can use some Security machines, such as the SecTechs, and can buy standard Security loadouts from the Security Weapons Vendor, and they have Security Training, and get doubled HP recovery from eating all types of donuts.
Vice Officers are a step up from Security Assistants, in that they're actually full members of Security, but they're a lot more laid-back than most Security personnel (not least because the actual level of crackdown on vice depends in part on the Head of Security, who may not care.)