RAM upgrade is not dependent on the amount of VRAM your graphics card has.
It's dependent on how much RAM your laptop supports, which for the Asus ROG GL553VW is 32GB maximum.
Your laptop only has two slots for the RAM, and as one of these is already occupied by an 8GB stick as factory standard, you wouldn't be able to add two more sticks, you'll only be able to add one stick.
If you want to install the maximum amount of RAM possible, you'll have to discard the existing 8GB RAM and buy two 16GB sticks.