Your GA-Z68MA-D2H-B3 has 4 240pin so you have 4 memory slot. A set of Corsair Vengeance comes with TALL heat spreaders. You should install the memory first before installing the heatsink. Some people installed in the first slot without any problem but the heat spreader will be close to the Hyper 212+. Don't think you will have any problems though. If anything, you can use the other RAM slots.
I can install the memories in the other 2 slots without any performance issue? If Im not mistaken there is some advice to install it in the first 2 slots.....
Where are the other positions i can mount the fan (sorry im really noob hehehehe) ? Everywhere I see the coolers fans over the ram slots......probably because thats is the best position I really dont know !
It is the best position since the air that flows through the cpu cooler blows toward the case fan that takes air out of your system. You don't have to have RAM with such high heat spreaders unless you plan to overclock your RAM.