Safety-critical systems are well documented and standardised (e.g. IEC 61508, RTCA DO-178B) within system design cycles. However in Defence and Security, systems that are critical to the success of a Mission are not defined within the literature nor are there any guidelines in defining criticality in their design or operational capabilities. When it comes to Vetronics (Vehicle Electronics), a mission-critical system, is a system with much complexity and mixed criticality levels that is a part of the overall platform (military vehicle) offering integrated system capabilities. In this paper, a framework is presented, providing guidelines in designing efficiently and effectively mission-critical systems considering principles of Interoperable Open Architectures (IOA), mission-critical integrity levels and following new standardisation activities such as NATO Generic Vehicle Architecture (NGVA). A Defensive Aid Suite (DAS) system is used as a case study to illustrate how this framework can be exploited.