In this study, we consider a single server vacation queueing
model with optional bulk service and an un-reliable server. A single
server provides first essential service (FES) to all arriving customers
one by one; apart from essential service, he can also facilitate the
additional phase of optional service (OS) in batches of fixed size b( ≥
1), in case when the customers request for it. The server may take a
single vacation whenever he finds no customers waiting in the queue
to be served. Moreover, the server is subjected to unpredictable
breakdown while providing the first essential service. The vacation
time, service time and repair time of the server are exponentially
distributed. The steady state results are obtained in terms of
probability generating function for queue size distributions. By using
the maximum entropy analysis (MEA), we derive various system
performance measures. A comparative study is performed between the
exact and approximate waiting time of the system. By taking the
numerical illustrations, the sensitivity analysis is done to explore the
effect of different descriptors on various performance measures.