but what is the exact aerodynamic answer for that?
The term 'exact aerodynamic answer' is, in itself, an oxymoron!
The fact is, as measured during flight test with a Decibel meter, repeated on more than one aircraft probably, with the VG's location being varied somewhat, 10 VG's mounted in that specific location on the glare shield reduced the noise by 3 or 4 db, as empitically proven. Game over, go solve the next problem.
Establishing the 'why' is not likely to reduce the cost or improve the performance, or even be useful on another airframe of different configuration!