Hollister is inexpensive, so I wouldn't consider that vain at all. However, I teach high school in a suburban school and Hollister and Abercrombie is what the kids I teach wear, and it was all the frat boys wear in college, so it's a little young-ish, in my opinion. (Although I'm not sure how old you are so that may suit you.) But even if you were wearing all designer stuff, that wouldn't be vain, it would be hot...as long as you don't spend more than me

(That's hard to do though!)
If dressing preppy is vain, vanity is my favorite sin