A 45 degree attack is good for accuracy, but dangerous when the GV can shoot back. For Flaks, a near vertical attack makes it harder for them to track you.
Not sure what the minimum size bomb is to kill a Panzer, but I wouldn't use anything smaller than 500 lbs (250 kg). If you have time to rearm, and want to be real sure...two 1000 lbs.
You have to be 1.5k to see the GV icon, but the vehicle itself is visible much farther away. I spot them, then wait for them to get near a landmark terrain feature, to make it easier to re-acquire when rolling into a dive.