I always though that pilot rammed Vader on purpose. He was fired upon, screamed look out at his leader and did his best to protect him from incoming fire. At least that's what I've always thought since watching ANH while I was around 4 years old.
As to why Han didn't shoot at Vader... it's Han, really. He went in, guns blazing wildly, screaming like an idiot when nobody besides Luke (and maybe Rebel command) could hear him. It's luck he actually did hit something. :)