Vivid agents [48] are software-controlled systems whose state comprises the mental components of knowledge, perceptions, tasks, and intentions, and whose behaviour is represented by means of action and reaction rules. We present an execution model for vivid agents which is based on an architecture for concurrent action and planning (CAP). We evaluate the concept of vivid agents and the CAP architecture in distributed diagnosis including fault-tolerant diagnosis and the diagnosis of an unreliable communication protocol.