Learn the stack and priority. Yes, this is part of any game of magic, but in vintage especially knowing how the stack works (when do triggered abilities go on, how does a spell resolve etc) can make or break some situations.
Know how delve, phyrexian mana and other alternative costs work with trinisphere and sphere of resistance type effects
Game 1, unknown opponent, if they quickly mulligan below 5 without much time considering their hand - they are on dredge
speaking of mulligans - learning how to mulligan properly is a highly useful skill
Remember - the person you are sitting across from the table at (at least in paper) is a player you WILL see again in a tournament. Pay attention to what they play and how they play it. And treat the person with respect and be friendly.