The Discord Missile is launched at the beginning of the engagement phase (as opposed the system phase for proton bombs/proximity mines etc, or in the end phase in the case of the DRK-1 probe droid).
Also the card from the Discord Missile does not use the keyword "attack"
Does this imply that the Vulture droid (or presumably the Hyena droid) may still make a primary attack during that engagement phase, despite having already launched a missile?