How Good is VAC

VAC is neither good or bad, I would say it is average anti cheat software, VAC will stop common low level cheats and scripts but most of the time premium hacks go undetected.
VAC can ban premium hacks but hacker must be stupid, if hacker use hacks carefully %99 of time hacker will not be detected.
I know some hackers banned after 6-9 months later so if a hacker can hack for half year without getting banned i would say VAC is not good at detecting smart hackers and it is hurting the game.
If they want to improve their game they should invest money for better Anti-Cheat they can use Machine Learning, Advanced AI, better Algorithms to detect cheaters or even buy premium hacks
from cheaters when a cheater release their plugin you will update the game 4-6 hours later than detect any hacker using new version of cheat etc. than ban cheaters each week by that way
you can stop most of the cheaters but Valve does not care about cheating problem of CSGO, Their VAC Anti Cheat is 6/10 not good or bad just average.