the shock levels, based on your CYA levels, will not bleach your liner- 25 ppm chlorine without CYA would be bad for the vinyl liner, but with your level of CYA is not bad as the CYA has it locked in place and limits its reactivity (more or less- chem geek can give precise scientific details)
the level of 20, while maybe not completely ineffective, would prolong your efforts and ultimately be "wasting" bleach- it will take you longer and more effort/money/bleach than if you achieve the suggested levels (and it could be completely ineffective- each pool and situation is unique)
The only way to get rid of excessive CYA is to partially drain and refill your pool- the CYA doesn't react with anything to disappear and under normal circumstances just stays in the pool!
You might, at your level, consider a partial water replacement- 100 is the max level most CYA tests show; your water could be 100, or could be 200 and only shows 100 on the test. Even if your water is truly 100, not higher, the amount of bleach required gets frustratingly high with those CYA levels.
The formula for bleach is simple- whatever % bleach is the ppm per 10k gallons; 6% bleach will give you 6ppm in 10k, 3ppm in 20k, etc...