Yep. Hypercomm transmissions can't go through shields, so in order for Vader to be talking with those ship captains, their shields had to be down. Needa also had to order shields raised after coming out of the asteroid field when the Falcon turned to ram them. Interestingly enough, in the novelization, several large asteroids impact Avenger's hull and do absolutely no damage. Do the Tyranid ships have to actually get up to ships to board, or do they launch boarding parties from a distance? Because the distances we see ships fighting at Coruscant and Endor is considered point-blank and, according to Ackbar in the ROTJ novelization, so close that the crew isn't even trained to perform under those circumstances.