Paul Bettany is the perfect person to play Vision. He has this baby face and manly face at the same time. Kinda like he was birthed as an adult. I know that sounds really weird, but it's hard to explain. His skin is very smooth and his eyes, nose and mouth make him look somewhat innocent, but his bone structure, especially his jaw and brow, has a very rugged look to it, just like Vision in the comics. He looks like he could be an android already without the make up. He also has a soothing voice that has a deepness to it, but not too rough. Plus his accent and the way he talks makes him sound wise and a little bit monotone.
I can't express enough how perfect he is for the role. It blows my mind how convenient it was to get him considering he plays JARVIS. It even works plot wise. The stars aligned for this one.