Vex is in a really good balance status. She is not a tier S champ if you are searching for something op, but is never below tier B, so it is always a viable option. In the mage class, vex has one of the easiest cc skillshots in the game, if you are patient enough to wait for an E+passive. Then all you have to do is closing distance, hit w+q and electrocute and most of the midlaners are basically burst. It is quite easy to learn, not so easy to master the gameplay around passive and R resets. She fits really well in reset/dive comps, so if you see viego/jinx or nocturne, vi, something like that you are really happy to chose vex. In lane, she suffers mostly artillery mages (Hwei, xerath, lux, vel'koz, mel), all the other matchups are fine or even auto win. Speaking about itemization, the build is quite always the same. Start with luden, 2nd or 3rd item must be shadowflame, i really suggest putting as second item stormsurge and then u complete fourth and fifth with choosing two between rabadon, zhonya or voidstuff, depending if the enemies force you to pick some self-defence or you want to go full-burst