Well as someone already said you can fight random teams and some mixed tiers on Vermillion, and win using your skills ( for example im still trying to defeat a OU team with my NUs), so is not 100% predetermined.
But on officials events ppl is usually using the same pokes (best stats+ movesets), so you can predict almost the 50%(or more)of every team,so IMO is all about predictions with a little of skill powers (plus a good team building).
So you can always keep doing Vermillion fights to get that "surprise" factor on every team you fight, and enjoy some variety cuz wynaut