ARMA does this by default, though it doesn't account for ghillie suits. But hiding in vegetation definitely works.

I did not exactly know about that, I always thought the vegetation only affected the player's concealment (towards AI) if the vegetation was directly betwen the player and the AI.

What I had in mind would be more to simulate AI having a clear direct line of sight towards the player, but they don't "see him" because he is around so much vegetation while wearing adequate camouflage

That is actually the only thing that matters. AI in vanilla Arma can usually see you perfectly through any amount of grass or vegetation, but will decrease its detection range if you are touching it. The way it works is this:

Twigs and tree branches and the like will theoretically obstruct vision, but only if the mesh map completely occludes your spine and head. Some 'bush' objects have a more or less solid hitbox, while some have a skeletal one. In both cases, the AI vision blocker does not match and is generally much smaller than the 'plant' graphic. In practice that means that having plants of any type between you is less valuable than just trying to perfectly center yourself on a lamp post or street sign, because the 'skeletal' hitbox will almost never perfectly cover every pixel of detectable area. If a single pixel is considered visible, the foliage provides no advantage against detection whatsoever.

Grass has no hitbox, so it's just the decrease to vision range which stacks with your uniform's 'camo' rating. If you submerge yourself in a bush that happens to have a solid hitbox or go motionless while prone in grass and wearing a ghillie suit, the AI will effectively be unable to see you even if they run into you.