As electric vehicles (EVs) are currently under-utilized, the features of deploying EVs as distributed energy resources (DERs), based on an electric vehicle as a service (EVaaS) framework are exploited and a resource allocation scheme is proposed for optimum association of dispersed EVs with critical load for demand fulfilment in microgrids. The proposed approach is based on a combined economic emission (CEE) optimization model where both energy costs and carbon emissions are taken into account. The CEE optimization problem is then formulated as a bi-objective optimization problem, considering a number of practical constraints, such as energy demand, cost budget, emission limit and charging station limit. Carbon price is introduced to convert the bi-objective problem into a single objective function. We included EV battery degradation cost to ensure EV owners are not worse off after EVaaS participation. The feasibility of the proposed model is demonstrated in simulation studies. The approach has been extended to evaluate the trade-off between EVaaS and conventional DERs. Numerical results demonstrate the efficiency of the proposed resource allocation scheme.