There are definitely multiple groups. What your experience is like will depend on what your interests are - unlike high school, if you don’t share interests/aren’t similar to one type of person, you will almost never come into contact with them the entire 4 years (besides maybe like the first week of school where people are meeting each other in the freshman dorms, and the first week of orientation with people in your orientation group). What you experience is will basically be self-determined.
If you like partying and knowing a lot of people and the work hard/play hard lifestyle, the greek scene is definitely present and strong on campus. If you don’t, you can have a social life without ever stepping foot on greek row. There are definitely alternative communities like mcgill (basically a hippie dorm), wilskills, or christian groups like BYX that don’t drink. There are also plenty of more tight-knit friend groups that maybe have small dorm parties or just stay in most nights, study a lot, play league of legends, explore Nashville etc. (a lot of engineers and international students).
Alcohol and marijuana are obviously everywhere, just like in most every other American university. Having the stronger greek life scene might make it slightly more prevalent than at quieter/more intellectual schools (i.e. somewhere like UChicago). As far as other drugs go, coke is fairly common within the upper tier of frats/sororities. People more into the drug scene might occasionally use LSD/mushrooms/molly, probably a minority of people in the frats and in hippie-like GDI groups like mcgill. I wouldn’t say that there’s any peer-pressure to use, but you won’t fit in so well in certain social settings if you don’t, through self-selection (i.e. if you never want to drink, it’s less likely that you’ll enjoy frat parties/pregames, although there are frats/sororities that cater more to that type of person. If you never want to use coke, you might have less in common with wilder frats/srats and be less interested in joining).
For dressing/looks, there definitely isn’t any sort of campus-wide pressure. (Although people seem to tend towards more conservative/preppy styles if anything, rather than streetwear/other newer styles you might see in big coastal cities. I’d say that’s out of ignorance for any other style rather than preference/pressure though). In certain subgroups there probably is a stronger sense of how you should look (i.e. if you want to be in a top sorority where all the peer members care about appearance, you might care about appearance more as well. If you have a bright purple mohawk and dress super avant-garde you might not fit in so well there, but if you’re that type of person, would you really be likely to rush that sorority anyway?)
Overall, I’d say it’s pretty similar to any peer elite private school. A lot of these questions you have, like drug use, appearance pressure, partying/no partying, will be standard at most places - most universities (non LACs) are big enough that they’ll be populated by pretty much the same distribution of people/interests. What I do think is important to consider though, is a sort-of preprofessional/intellectual cultural continuum within the top 20, where Vanderbilt is probably at the preprofessional end (maybe along with schools like northwestern, duke, notre dame). Even students who are extremely smart mostly want to come to class, get their A’s, go home and party/enjoy their free time, and get their acceptance to med school/law school/goldman/deloitte. If you’re seeking a social life that’s an extension of the classroom, you can find it here, but it’s a lot harder, and I might look elsewhere first.