The facts are that the possibility of death is always present and meditation is probably one of the safest place to be (opposed to walking down the street, etc..).
Now, in vipassana meditation practice, you observe what takes place, so when that fear of death arises, notice it and, if simply noting does not make it disappear, note that as well and examine the feeling very closely in all its aspects. Simply make that feeling the object of your meditation. It is most likely that progression will get you to notice the arising of the feeling earlier and earlier, and after a while you will catch it early enough so that it will not develop in a panic attack.
Breath disappearing is not at all unusual, it is one of the possible good sign that progress is being made. On a physiological level, the breathing becomes very slow and very, very subtle, so that you barely, or even not, feel or notice it, but it is there and it does supply a sufficient amount of oxygen to the body which is not exerting itself, is untypically restful, and thus needs very little.
Given the progress you are obviously making, It might be very good to try to have a talk with an experienced meditator bikhu, if at all possible.