Vortin is a diamond merchant. This means that he owns a warehouse full of portable, hard-to-trace, valuable little things (in Ankh-Morpork!) This warehouse is a favourite target of thieves of all kinds.
Not to be confused with Vorbis, this is another of those non-speaking characters who runs a very profitable diamond and gemstone trading business in Ankh-Morpork. First introduced in The Colour of Magic, the name and business of Vortin recur throughout the books, often in the context of the Watch either investigating, or thwarting, a break-in to a business that must need VERY good security.