‘Altvac’ is short for ‘alternate vacation.’ It’s one of the things that you get when the concept of alternate timelines is first finally proven, then commercialized: people go find a timeline where they exist, only in a different life, and then they and their alternate selves swap timelines for a week or two. It’s considered rude to not tell anybody that you’re temporarily living your alternate self’s life, but it often happens, which can lead to some very interesting social situations. At least one media company films the most entertaining ones, for later commercial use.
Why would anybody do this? Well, most people don’t do this: altvacs cater to a very specialized market. But, well, that’s true for most extreme vacations, too. Some people like the idea of spending a couple of weeks meeting old friends for the first time (this very much includes spouses, although the promotional literature never spells that out). Other people like to find out once and for all whether they really would have been happier if they had kept up with that band, or whatever. And some people just want the chance to get away from demands on their time, which is really the reason for vacations in the first place.
Fortunately, most people don’t try to use altvacs to commit crimes. After all, they’d just be making like difficult for the person who actually lives in this timeline. Being the victim of a crime, however? That’s a different story. And a complicated one, if the person committing the crime doesn’t realize that he’s targeted somebody on an altvac.