Adding vision panel to fire door

That depends on the fire door leaf manufacturers requirements.  Many fire door manufacturer's insist that such work is carried out by approved processors under licence.  If the vision panel is not fitted in this way, then the fire performance certification for the door is void.

However, if you are fabricating a door from a fire door blank, you can cut and glaze vision panels so long as the work is carried out within the scope of the blank manufacturers instructions.