Vader himself can't multitask quite as well as he normally would back home. He's grown somewhat reliant for his helmet's input and display to make up for his lack of hearing and vision. Something he doesn't have and sorely misses. That being said, even though he is focused on the dragon he is still keeping a feel out for where that mage is and for where Tech is.

And he doesn't have time to throw a Force shield around Tech in order to protect him from the blast of fire. What he can do is shove an arm behind him in order to fling Tech himself out of the way. Hopefully he'll be able to catch himself in landing but it's not something Vader can spare the attention for at the moment. Even if he shoves him hard, he doesn't shove him too far off the ground.

One or the other of the that's needs to go down.