I completely understand what you are saying, but there is a second reason you trade up. Regardless of how said talent compares to their peers in the draft, if a player will take a unit to another level, you consider it.
Example:
Zeke Elliott is the perfect illustration of what I'm talking about. Granted, you could argue he was also a top 5 talent in the 2016 draft, but for me that pick was more about what he would do for Dallas in particular. I'm not saying Zeke wouldn't find success elsewhere, but pairing him with that offensive line made all the sense in the world.
I think of Vita in that same light. Vita could easily disappear if he is drafted by a team that is in severe need of dline help. But if the Cowboys draft him, it could be devastating for the Cowboys opposition in 2018 because pairing Vita with DLaw and DIrving could very well be unstoppable. So in that light, it's not about how Vita compares to his peers but moreso how much better the Dallas D could be with a true 1-tech, who penetrates and moves off the ball like a 3-tech. It would be ridiculous. So, yes, I'd absolutely consider it for the right price.
The problem is my right price is no more than the Cowboys first and a fourth, because as it stands right now, the Cowboys need to also address Offensive Guard, Safety, Linebacker and Swing Tackle. The Tackle could be found in the midrounds, but unless they address it in Free Agency, the Cowboys will be handcuffed to finding an OG, Linebacker and Safety early.