The Vex build for Mid is Luden's Tempest and Electrocute. This LoL Vex guide for Mid at Platinum+ on 13.19 includes runes, items, skill order, and counters.
53.2%
(+0.4%)
50.9%
(-0.3%)
5.1%
(-0.5%)
6.9%
(+0.2%)
854
(-1)