I guess I would have to pick Tannehill because he did throw a costly pick, but that would be like giving the shame of the classroom to a student who got an A- because everybody else in the class got an A.
Well, he threw two picks. One went off both hands of Cameron, the other was a total miscommunication. I'm assuming your reference is to the latter, but we have no way of knowing if Tannehill or Cameron (he was the target again, IIRC) was the one who didn't make the correct read.
Tannehill was extremely precise for most of the game, delivering some long strikes at important times. I would like to see him throw the ball away on a few of those sacks, though. And it wouldn't hurt if he had the awareness to spot obvious defensive offsides penalties and just chuck it downfield for a free shot instead of running the ball out of bounds for no gain or handing it off fore a pile-pushing 2-yarder.
Vernon committed a costly penalty, so maybe he would be a better choice.
Again, Vernon committed two roughing the passer penalties. Titans got exactly zero points off those two drives, so neither turned out to be costly. Both were on third down, though, and gave the Titans a new set of downs. I understand why Vernon drew the flag on the first, but if you look at the play, I don't really blame him for trying to dive for Mariota. Not sure what happened on the second one.
Overall, I think the shame of the game has to go to the secondary. Grimes played quite well, but everyone else was a step behind (as usual).