Overview- Viper is not a textbook type of character to say the least. She has great zoning tools and dangerous rush down. Adapting to your opponent is the only way to play this match up correctly. Chun Li must stay moving in this match up to avoid seismo pressure which is one of the reasons why this fight is bad. Jumping gets Hp. thunder knuckled and or Far Hk on reaction. You want to rush Viper down and defeat her before she defeats you. Since zoning is eliminated out of the equation do the next best thing, "Rush That Shit Down"-Valle. Chun's footsies are better than Viper so pushing her to a corner won't be all that hard, but keeping her there may be tricky. Standing outside of her C. mk range is important to whiff punish anything she does with St. mp or C. hk. At the same time preparing to punish her with a jump back Hp on reaction if she trys to burn kick out. Learning OS's on Viper is needed if you want to stand a chance against her. You will need jab fierce os and forward throw os that I will go into detail later about. C.mk thankfully goes under Hp thunder knuckle and burn kicks when timed right and is a good meaty to put on her wake majority of the time. C. mk on reation to burn kick are so important to avoid being abused and put into another guessing game. Avoid the situation all together. Well timed Hazanshu's beat seismo but can be baited by faint easily and be Dp or hit by ultra. Making Viper block and constantly mixing her up with throws and frame traps will change the tide of the match up drastically though.
PROS- Your normals are better than hers
C. mk goes under thunder knuckle and burn kicks
Chun's OS's limit alot of her options
Hazanshu beats seismo
CONS- Her overhead beats alot of Chun's normals and even ex SBK if timed right
Your zone game is eliminated
When she has bar be cautious of your block strings and trying to play footsies
Seismo chain pressure hurts 900 health.lol
Option Selects- There are two very important OS's that I will describe.
C.lp~St.Fp - This OS beats BurnKick,BackDash, and ExSeismo. Loses to Hp. thunder knuckle and wake up ex Fient. Do this after an untechable knockdown. Press C.lp then St.lp and St.Fp at the same time. If they block you should get 1 jab. If they do one of the above things first mentioned a fierce will come out.
Forward Throw~J. Hp~Sweep- This OS beats BurnKick,BackDash,ExSeismo and Hp. thunder knuckle. This OS only loses to Fierce Fient on wake up that was discovered by PJS Latif to have slight invincibility on start up. Here is the video where I explain this OS.
Ultra of Choice- U1- I see some Chun's use this for the punish of seismo or whiffed burn kick. Viper falls out in the corner though. It does limit some of her options, but I personally us U2.
U2- Can combo from AA or on ground and fits better if you are rushing Viper down to get quick damage.
AA- Your air to air game is better than her. Utilize neutral jump Hk,Hp and forward and backward jump Hk and Hp. Air throw works wonder also.
BURNKICK"The Horror"- Many of you, I know myself included get frustrated from mashed out burnkick during blockstring. The way im of recently dealing with them is C.lp~St.Hk frame trap easily blows up burn kick and does decent damage in the process. Reacting to the burn kick with C.mk helps to make it whiff sometimes if delayed will hit them also. St.lp will hit burn kick but isnt enough damage to make them fear doing it. Learning to punish mashed burn kick will help out not only the match up but your reaction.
Another note about thunder knuckle block strings. Since she is at negative dont hesitate to press a button, but remember you can be Dp'ed so C.mk is not a bad option after blocking thunder knuckle. Also thunder knuckle strings can be Ex SBK because there is a gap. Without bar though you are forced to block. Beware that you can be baited and punished Horribly.
Hopefully I helped out in this rough match up. I have alot of practice in this match up so I love spreading the knowledge. Please comment and questions. Your feed back is important.