The Vex build for Mid is Luden's Tempest and Electrocute. This LoL Vex guide for Mid at Platinum+ on 13.23 includes runes, items, skill order, and counters.
Vex Counters
53.2%
(+0.8%)
51.3%
(-0.6%)
5.1%
(+0.6%)
5.8%
(-0.5%)
854
(-30)