Delay tolerant networks are challenged networks that are characterized by intermittent connectivity that randomly impacts end-to-end performance. One way to improve the quality of communications is by routing data (i.e., bundles) through more than one channel or end-to-end path whenever the case permits. We analyze the probabilistic optimal routing problem for this case through the formulation of a queuing model with vacations that serve to represent the transmission of data bundles over channels of sporadic availability. Despite transmissions could be scheduled in some cases (e.g., knowledge of the expected contact plan can be known in space networks) we assume unpredictable transmission opportunities where it is not possible to establish the sequence of future contacts beforehand. This makes the analysis applicable to different kinds of delay tolerant networks. For example, with the Licklider Transmission Protocol, when a disruption occurs in the midst of a bundle transmission, the service can be temporarily stopped and restarted at the next contact opportunity. A numerical solution of the model illustrates the trade-offs that exist in the optimization of bundle flows over multiple space channels.