The definition starts with the assumption that the aircraft is hit.
Susceptibility is that part of aircraft survivability that deals with avoiding hits.
This information is used as a basis for design changes to reduce vulnerability and for assessing the results of actual encounters with the threat in higher level simulations. Endgame is that part of simulation that deals with the last few milliseconds of an engagement between and aircraft and threat. It includes the evaluation of direct hit, blast, fragment hits, etc. The A-10 picture is of a simulated encounter with a missile, showing fragment hits on the aircraft. Vulnerability data are used to assess the endgame results.