Network Function Virtualization decouples network function deployment from dedicated hardware and reduces costs. Network services are structured as chains of VNFs. Each chain is a set of VNFs that should be executed according to a predefined order. For some applications, VNF chains should be executed within time constraints to meet the application's objectives. Most studies provide a solution to allocate substrate network resources to the chains without considering admission control. Allocating resources to all chains may not be possible due to resource limitations. Efficient admission control is therefore required to determine chains admission. This paper proposes a joint admission control and resource allocation mechanism for VNF chains. We propose a resource allocation mechanism based on the idea of parallel VNF processing to meet tight time constraints. As the used assumptions in deterministic modeling of the system do not hold in a wide range of network conditions, we propose a stochastic modeling at which VNF chain execution is modeled by a Queue network. The Queue network is analyzed to calculate the expected value of the probability of deadline meeting in chains, according which the joint resource allocation and admission control problem is modeled as a non-linear optimization. The proposed optimization framework maximizes the profit of the network provider while keeping the confidence level of deadline-meeting for the admitted chains at desired levels. To have an efficient power usage, power consumption is also considered in network provider profit calculation. A heuristic for the joint resource allocation and admission control of VNF chains is proposed. The effectiveness of the proposed method is demonstrated through simulation.