They can be easily detected, but that really relies on the hack abilities and how secure it is. Expert programmers/scripters can make a very secure, and reliable hack, but still, if one feature uses "write to memory" type of exploit/hack, it will be detected probably instantly, but that also depends on the anti-cheat system, but since we're talking about VAC, it's kinda instant if it's write to memory type tool, but most of the hacks aren't, but you know, if a new update comes out and you're using outdated hack, it probably is detected and not safe to use.