I'm ready to give up on Moniak, he just looks awful, like he can't see a breaking ball's spin at all, and he lacks plate discipline as well. It's one thing to struggle, it's another to look like a pitcher trying to hit. I just don't see any flashes that might give you hope there's something there to build on, unlike Stott.
Vierling on the other hand had top flight exit velocity last year, and while he struggled out of the gate this year, looks like a major league hitter. Has the speed for center, may need some time to get more comfortable with angles, but isn't a liability out there. Given the options, I think you have to play him every day and see if he can handle the job.