I’m actually very familiar with the Bison matchup, so I’ll mention some key things…I hope others add or correct me if I’m incorrect in my assessment as well.
Against Bison:
A Bison without meter is pretty much a sitting duck, if you find a Bison crouching without meter don’t be afraid to jump-in with a j.hp, and if they’re hit by it follow it up with cl.hp > cr. mp xxx EX FBA, if they block the j.hp then go for a grab (for some reason lots of Bisons think that Vega won’t grab them).
When Bison doesn’t have meter and you jump in, he’ll either (1) teleport or (2) simply block what you’re attempting, making it easier for you to mount some minor st.lk > tick throw pressure.
If they teleport follow them with a lp. RCF or if possible slide towards them, it depends on how far they are that determines what options you have such as cl.hp > cr. mp xx EX FBA, or simple throw if they teleport right next to you.
I also think that Vega’s air grab is one of his most excellent tools in this match-up because Bison’s jump speed is pretty horrible, if you predict or clearly see a headstomp coming, don’t be afraid to air grab the shit out of them. Vega’s air grab beats a lot of Bison’s jump attacks. Make it clear in this match-up that you dominate the air, because on the ground Bison can really mount pressure up close.
You can punish his slide with EX ST because the recovery on Bison’s slide is pretty bad or cr.mk / cr. mp xxx EX FBA
If Bison has meter, know that they’ll usually use EX psycho crusher on wake-up, so do your best to bait this out by walking nearby but be prepared to block and once you block it you can punish it with slide or with a properly well timed U2 (if you have it)
But really on wake-up (IMO) is where Vega kind of sets the tone in this match-up because Bison doesn’t really have an anti-air or great wake up options…if he has meter bait out the ex-psycho crusher, if they ex head stomp and you bait it out, air grab them.
If Bison has you cornered and is mounting on pressure with cr.lk and scissors, simply relax and block and wait to reversal with a hk.ST or EX. ST on the recovery of a close scissors, if all else fails do your best to wall jump out of the corner…you can also keep him off you if you keep using st.lk after scissors just to mount on some block stun or even potentially stuff a scissors.
I also think that Vega’s overhead (Piece of Mercury) is very overlooked in this match-up. If Bison is crouching, and their playstyle is rather lame or less offensive you can actually force them to stop charging so much by simply throwing a PoM at random times that you feel he’s just sitting there and follow the PoM with a throw or jump back hp. Personally if a Bison isn’t responding much or becomes a bit more defensive I like to do st.lk, st.lk, PoM
Don’t be afraid to corpsehop > throw on a meterless Bison either.
I think that the positioning in this match is really supposed to be mid-range at combat (poke him out with st. mk or cr.mk if he walks forwards and air grab him if he tries to jump) and clearly up-close on Bison’s knockdown so you can mount grabs on top of st.lk , corpsehop > grab/cr.mk, baiting EX PC/Headstomps on wakeup.
U2 is the way to go in this match-up simply because of Bison’s jump speed which is so slow that it gives you enough time more than any other character to severely punish it with AE’s wack ass U2 start-up.
I hope some of this helped…feel free to ask questions or criticize, etc…