In this paper we propose a comprehensive technique to reduce the operational expenses (OPEX) of virtual network systems, which comprises the cost of using resources, the cost of requesting the underlying infrastructure to make changes to the allocated resources, and the cost of dropping network traffic and hence disrupting the service. We demonstrate that it is feasible to reduce OPEX by optimizing resource assignation without relying on thresholds, neither for determining the amount of resources to be assigned nor to assess the correct operation of the control system. Instead, our proposal relies solely on the context definition (day of week, time of day, current resource amount, etc.) and the events reported by external detectors. Finally, we quantify the benefit of considering such events when taking management decisions and how they are useful to step further towards the optimum assignation of Virtual Network Function (VNF) instances to accomplish the demands of the overall system.