The thing with Vlad is that while you are right, there is no area in which he is average or better (you could argue he is roughly average at inducing weak contact perhaps based on his stat cast numbers) he has also not been really bad at anything. His whole package is pretty consistent with that of a back of the rotation starter who can fill some innings while being unspectacular but not terrible.
Vlad has a wide array of mediocre things going for him, he has 4 pitches that he throws quite a bit. His 3 off-speed pitches are effective and his fastball is not. He is basically a full-blown junk baller. I just don't see how you can bank on his stuff "playing up in the pen" so what if Vlad is throwing 95 instead of 93? Whereas Santillan might be able to get up to 99-100 with the potential for a very good slider in the pen that he can throw 50+% of the time if need be in short stints. Santillan has a shiny K rate but is pretty rough everywhere else. He walks too many, and gives up too much hard contact. Santillan might be on and give you a great outing here or there, but he is probably going to be having short and bad outings more often than not. Hoffman just kinda all around stinks, I know the first time through is getting referenced but that's largely smoke and mirrors. He has a BB /9 of 6 the first time through which is just unacceptable for someone with his caliber of stuff. The difference between his first and second time through is largely just BABIP.