Ahri vs Vex Mid Matchup
Ahri Mid wins 47.9% of 3,442 Low Elo games against Vex Mid, -1.0 points vs expected, 2-patch window 26.19-26.20.
Researched on LoL patch 26.20. Tips are written to hold at any rank; stats come from Low Elo ranked games up to patch 26.20.
How to play as Ahri
Watch Vex's fear indicator: when it is ready, use Fox-Fire (W) to sidestep her Q or E and trade after she spends the empowered spell.
Ahri's Spirit Rush (R) is a series of dashes, while Vex's Doom 'n Gloom passive marks nearby enemies when they dash and can empower her next ability to fear.
Ahri's Charm(E) immediately stops movement abilities, while Vex must recast Shadow Surge(R) to dash to its marked target; Ahri can interrupt that approach with her charm.
How to play as Vex
Keep Doom 'n Gloom's fear ready when Ahri has Spirit Rush (R) charges; her dashes can trigger your passive mark, and the fear can stop her follow-up.
Sources
These notes come from r/AhriMains and r/vexmains, checked against the champions' abilities.