ok so i solved it. It seems that after all it was the vbga cable that caused the problem.. not sure about the length, cause i tried another one (3m instead of 5) and it also didn't worked . BUT with one 1,5 m works PERFECT. so, except the length, the three cables differ, in that way : the two cables that dont work, seem to be missing one PIN that the other one has. i dont know what is the case... but it works now!