Here's a simple but unsatisfying answer for you: It's not detected yet because I haven't made the detection yet. I haven't made the detection yet because this problem hasn't been my top priority.
The reason why Vanguard doesn't prevent this is because technically this is how graphics drivers are meant to work. It's a legitimate feature but not one we want to support on Valorant. Vanguard proactively protects against cheats that plan to modify the game rather than stuff like this.
Instead of making the detection I'd rather fix the underlying issue in game. If this becomes more popular or a fix will take longer than I'd like then I'll make a detection and ban all the people.
Anyone using this for competitive advantage should know that they can be banned for it even if I have to do it manually without an automated detection.