Virtualization technologies have recently gained a lot of interest in Grid computing as they allow flexible resource management. Grid'5000 (G5K) is a French national Grid platform used for computer science research to experiment all layers of Grid software. Computer scientists reserve G5K nodes prior to their experiments. In G5K some low priority jobs are executed in best effort mode on the node idle time slots when the latter are not part of any reservation. However, best-effort jobs may be killed at any time by the Grid job scheduler when the nodes they use are subject to higher priority reservation. This behaviour leads potentially to a huge waste of compute time or at least requires users to deal with checkpoints of their best efforts jobs. In this paper, we describe the design and implementation of the VMdeploy framework which exploits virtual machines for executing best effort jobs in order to solve the best-effort issue in G5K platform. VMdeploy manages snapshots of the best effort jobs transparently to their users and thus ensures the progress of these jobs avoiding most of the waste of resources. Results of a preliminary experimental evaluation are presented. While designed in the context of G5K, VMdeploy can be used in combination of any job scheduler in clusters and grids.