Vex vs Hwei Mid Matchup
Vex Mid wins 49.4% of 1,178 Low Elo games against Hwei Mid, -0.9 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 Vex
Stand behind minions but keep some distance so Hwei cannot line up QQ or EQ through the wave, and avoid standing in his QE damage zone.
Use Looming Darkness(E) to trade when Hwei is busy with the wave, and look for Shadow Surge(R) only after Hwei has spent his fear or is low enough for the all-in.
Vex's Shadow Surge(R) gives her a dash onto Hwei on recast, while Hwei's EQ fear can interrupt that follow-up if he keeps the control spell available.
How to play as Hwei
Keep EQ ready while Vex has Shadow Surge(R) available; fear her if she recasts onto you instead of spending your only reliable stop on routine poke.
Sources
These notes come from r/vexmains, checked against the champions' abilities.