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.
The other one, which is very niche, is when you have Survival, Bazaar, and 1 Green but no extra creature to go for ESG: at their EOT, pitch your guy to get Rootwala, then if things don’t improve after you’ve drawn, use Survival to discard Rootwala with Madness, with all that on the stack activate Bazaar, then resolve Rootwala and Survival, getting Hollow One. Now all your Vengevines can come back!
Most people don't look at demonic tutor as acceleration but getting lotus with it is +1 mana of any colour. Its sometimes correct to get an academy or sol ring if your low on mana.
Don't overplay your board (especially as a shops player). Hold some stuff in your hand just for mass board removal.
As a storm player holing a land in hand while casting dark ritual to bait them to Misstep the Ritual, thinking they can constrict you on mana. Withholding information is not really a Vintage specific thing, but some of the applications of this are.
Very Common: Hurkyl's recall yourself bouncing your artifact acceleration back to produce mana and storm count. Same works with Chain of Vapor ofcourse.
Casting Cabal Therapy or Thoughtseize on yourself sometimes has slight advantages: It might get a Blightsteel Colossus shuffled back into your deck if you want it as a tinker target. Pretty rare, but it sometimes helps: Taking a spell you are not planning on casting to get quickly to Threshhold for Cabal Ritual. It costs a mana and 2 hand cards to produce 2 more mana, quite the all in play.
Gushing in response to a Balance to make your opponent sac their lands.
@thecravenone Wait. Someone has actually resolved Goblin Welder in the last 24 months?? Pics or it didn't happen!
Well it was against a Hangarback Walker deck so it wasn't nearly as hard as usual
Based on the best streamers I watch like @The-Atog-Lord, @ChubbyRain and Brian Kelly, a thing which they are really good at is to always thoroughly consider their options before making a play and also to plan for the worse (because things can go horribly wrong quickly in vintage). Sounds easy in theory but so hard to do when you play yourself. Another important thing is to always have the old card-frame version of every card
You can block with a Mishra's Factory and use it to pump itself.
You can activate an Orchard on your opponents end step so they don't get an attacker the turn before you Oath.
(not explicitly vintage) +ing a planeswalker is a cost, so you can cast a Jace and + it if you think it will get bolted.
This plays out as you cast a Jace TMS, hold priority +2 it then pass priority. They then bolt it. Now your Jace is alive, at 2 loyalty and at that point the fateseal resolves.
I'll keep adding to this thread.
@kistrand To piggy back on this, plan for the worst case scenario, but don't always act like the worst case scenario is the case. For example, if your opponent is on blue, has drawn some and has 5 cards in hand - he MAY have FoW, so you have to carefully consider casting that clutch spell. But if you DON'T cast that spell, and have nothing you could topdeck to push the spell through next turn, then not only did you virtually give them FoW in their hand, but you also gave them time to dig more and actually find FoW.
Sometimes they have the stop, sometimes they don't. Sometimes you cast your spell and they counter and it seems crippling (but if you held it in your hand, they'd STILL have the stop, so does it matter?). If you never cast the spell out of fear of a counter, then they ALWAYS have the stop, whether a counter is in their hand or not.
Sequencing land drops and spells is a skill many still lack in mtg in general.
spell sequencing: For one, It took @Smmenen writing an entire book on how to play Gush properly before some people caught on to how good the card is outside of Grow.dec or Gush Storm. (Turns out, Gush was THE blue deck)
On lands: I still see players making the mistake of cracking one for a dual as soon as they play it, (many still think "thinning" your deck is a valid reason) unnecessarily giving away info and opening themselves up to wasteland.
Many times players lose to themselves, making the wrong land drop turn one.
@BranDawri Also, if two Oaths are in play and an imbalance of creatures exists, they will both trigger. At this point, you can respond with Forbidden Orchard creating a further difference in creature amounts. When the first one resolves you can evaluate whether you need to use the second Oath ability. There are a few iterations of how this plays out, but you get the picture.