EDIT: The math is wrong. See later post for proper Vex numbers.
How much are Vex and Graze really worth?
Without Vex, you will make every attack at your base hit rate. I'll assume 60%.
If your average damage per hit is 10, then you're doing, on average, 6 damage per attack.
With advantage, 60% becomes 84% (a 40% increase). While advantage is up, you're doing 8.4 damage per attack.
How many attacks will Vex last?
With a 60% hit rate, it will take an average of 1.67 attacks to land the first hit. With an 84% hit rate, you will land an average of 4 attacks before you miss.
Total damage is 1 x 6 + 4 x 8.4 = 39.6 over 5.67 attacks, for an average of 7 damage per attack.
Long term gain, assuming you don't switch targets, is thus +17%.
For comparison, Graze.
With just +3 Str on a 1d10 weapon, your normal average damage is 8.5. A 60% hit rate would thus give an overall damage per attack of 5.1. With Graze, you will also gain 40% of 3, or 1.2 average damage. That's a 24% increase in average damage. At +4 Str that increases to an average of +28%, and +5 Str gives an average of +32%.
Other riders on attack damage, such as Rage bonus damage or extra fire damage per attack, will reduce the value of Graze.
Comparing again at 65%:
Vex: Advantage is worth +35% damage. It will take 1.5 turns to land, and last an average of 5 attacks. That's an overall damage increase of 19%.
Graze: +3/+4/+5 Str with a 1d10 weapon at 65% hit rate will give an average damage per attack of 5.525/6.175/6.825, while Graze will contribute 1.05/1.4/1.75. Gain is +19%/+23%/+26%.
Comparing again at 70%:
Vex: Advantage is worth +30% damage. It will 1.4 turns to land, and last an average of 7 attacks. That's an overall damage increase of 20%.
Graze: +3/+4/+5 Str with a 1d10 weapon at 70% hit rate will give an average damage per attack of 5.95/6.65/7.35, while Graze will contribute 0.9/1.2/1.5. Gain is +15%/+18%/+20%.
Summary:
Broadly speaking, Vex and Graze are generating about the same average improvement to damage (somewhere around +20%). Graze becomes more valuable as accuracy drops, while Vex becomes slightly more valuable as accuracy increases. Graze also increases in value as your Str increases.
My new conclusion on Vex is that it's not overpowered, or even necessarily top-tier. My other math on Graze has led me to consider it a solid option when you don't have advantage, so now I'll consider Vex as being in the same mid-tier spot.