In regards to the Ion cannon, I always assumed it was a really high tech piece of equipment that was difficult to get hold of, especially considering the size of the thing. It's basically the ace up their sleeve in order to escape.

As for Bespin, it seems to me Fett was figuring out where they were going when tracking them, triangulating their trajectory, then instantly took this information to Vader.

Vader would then instantly move in as quickly as possible. By the time Han and crew got their on their limited propulsion Vader had already set up.