Didn't like it. It's got ill-defined characters, a sloppy plot and nothing much to recommend it, if you don't count some mild nudity.
Firstly, what is it? Is it a drama, a mystery or a skin flick? Answer: none of the above, because it doesn't reach the required mark on any count. Our protagonist, Su-Won Ji, has a sister who disappeared. We see flashbacks to when they were young. The movie then spends half an hour on middle-class couples having dinner together and I couldn't tell these people apart or work out who was married to whom. It's not that I have trouble distinguishing Korean faces (although as it happens, this time I did, a little), but simply that the movie doesn't bother establishing its characters.
Su-Won Ji digs up someone on the internet (Pil-woo Koo) who knows a story she associates with her sister. He goes by the name of Art Lover. The two of them meet up and there's some excitement at the halfway mark as people show up looking for Koo and claiming to be policemen. Ji and Koo run away. This wakes up the audience, but is never referred to again.
Eventually, the story coalesces. One picks up who's who by a process of osmosis, which is coming to be a weakness I associate with (a few) Korean films. Ji's husband is a cock, telling his wife that he's jealous of men who have affairs. They have a small daughter, who seems sweet but doesn't get much screen time. The important relationship in the movie is that between Ji and Koo, who don't much like each other but feel sexual urges towards each other anyway. Koo's motivation is that he lives his life by storytelling games, locking all his interaction with Ji into a framework of "it's your turn to tell a story now". Meanwhile Ji just wants to find her sister. She thinks Koo can help her. You could make a good mystery out of these ingredients, but this film isn't it.
So, is it trash that only exists to show tits? No, it's not that either. Don't believe the internet warnings that say, "Erotica/+18. WARNING: Contain nudity and Strong Sexual Content, this movie may not be suitable for some viewers." These people need to get out more. There's a long-ish sex scene at the end, but otherwise it comes across as a normal movie that very occasionally has a bit of nudity.
Does this contain anything interesting? Yes, here and there. I liked the animation about the goddess and the coyote's ambulatory penis. That was a bit different, at least, and it's sufficiently blatant in its symbolism that it might just get you looking for themes. Sexual relationships between men and women would certainly seem to be a strong element of the movie and not just in the obvious way, with interlocking marital infidelities, the horrific (to me) attitudes of Ji's husband and Ji's apparent response to them. Otherwise... nope, can't think of anything else.
Wow, I've already run out of things to say.
In fairness, it's not a horrible movie. It's just uninspired, with the script in particular being a swamp in which was drowned the potential for something clearer and more interesting. Much of the time it hardly seems to have a plot at all. It just burps forth scenes without giving the impression that they're going somewhere. The acting is merely passable, with Su-Won Ji having only a sketchy imdb filmography in which this is to date the last entry... although that said, she was in The Quiet Family (1998), the Korean original that Takashi Miike bent six ways from Sunday to remake as The Happiness of the Katakuris
Incidentally the title "Vanishing Twin" is a not particularly relevant reference to the phenomenon of twins that die while still a foetus in the womb and get reabsorbed. Would I recommend this? Hell, no. It's an obscure movie that deserves its obscurity. Even the nudity isn't worth your time. Su-Won Ji is an attractive lady, yes, but you'd have to be mad to call this porn. Instead it's simply what happens when movies are made by people who don't understand movies. Even a bad film would have been better than this.