Admiral Piett told him the Millenium Falcon entered an asteroid field and they couldn’t pursue it for obvious reasons. Vader told him to **** off and get the ship, whatever the cost. The Imperial Fleet then moved into the asteroid field and suffered heavy losses, as was stated by one captain. A Star Destroyer was even destroyed on screen (the captain’s hologram disappeared as he died). Who knows how many other ships were taken out in the pursuit.
Besides a little bit of black comedy, the point was that Vader was willing to go to any lengths to capture and torture Luke’s friends, even if it meant wiping out his own fleet.