It might make sense if the the TIEs were shooting at the asteroids and not at the Falcon, Vader wants the ship and her crew alive but the TIE pilots might need to clear a path through the rocky chaos to try and keep up with her (they do lose a lot of ships smacking into the things).

Possibly part of the reason why the laser blasts might not sit right is for the same reason why there shouldn't be thousands of fighters shooting the hell out of the Falcon as it escapes Bespin, Vader doesn't want them dead (he underlines this to the Bounty Hunters he hires).

Clearing obstacles is another matter.