The only difference I can see is the shape. The Tenacious has a dropped butt grip, and the blade is wider, which makes the closed knife wider.
I haven't had a Tenacious in hand - like many I made my observations based on photos and prior ownership of similar knives. I decided the Tenacious would not be as slim in the pocket and would interfere with getting my hand in for other stuff, especially in jeans, a preferred work uniform.
I bought the Vex. I also chose it because the Ti plating is extremely abrasion resistant and NOT black. I've been cutting double corrugated cardboard with it for weeks now, and there's almost no wear in the surface. It also has a very slick finish which contributes to cutting, and the flat ground blade is easy to choke up on or use full grip, an advantage with a flatter back profile. The blade back itself is rounded, not squared, and doesn't gouge my thumb as much under pressure.
The grips are a straighter profile, something I've used for decades. Dropped butts do allow the knife to rest more comfortably in the grip under pressure, but they also rotate the tip down in the cut. That may or may not be an advantage - it takes some time to experience a real difference, and a appreciation of whether you are actually cutting for that long anyway.
Both have opening holes, the Tenacious being more exposed. Some prefer that for speed of deployment, I prefer a slimmer profile in carry where it rides 95% of the time anyway. A few split seconds in retrieve and deployment don't make a difference when attacked by feral antifreeze boxes.
It seems a lot of reports come back that the Tenacious is more attractive or has better fit and finish, which those owners seem to value. I prefer a knife that actually works as a tool, and the Vex does that for me.