It's not caught by VAC. I changed the glow colour (which probably changed the signature), tested it a few months ago on local server and then played some Valve deathmatch on official servers, account is still good. I'd expect the game to at least throw me out of the server if I tamper with what objects should light up, but nope.
They do bans in waves to try and keep people from knowing what cheat/hack they used that got them banned. VAC might not have caught you, or you might get banned in one of the waves where they ban thousands of people all at once.
If I remember right, VAC doesn't necessarily immediately ban a cheater. It might instead put them on a list. After the hack's popularity increases, there is a mass ban of everyone detected using the hack at a later date.
If you changed a known hack you might be scheduled for banning at a later banwave instead. Otherwise cheat developers would just make changes and test what gets them banned.