The top picks not in lane to counter Vex include Kalista, Zeri, and Caitlyn. These picks are all good at countering Vex with win rates over 61%. Even if these Vex counter picks are not chosen in the same position as you, it's not a good idea to choose them into any of these matchups. They will very likely destroy Vex.
Yet, if the enemy team brings either Karthus or Singed, you should seriously think about taking them, as they easily wins against these selections with win rates over 55% against each ofthem.