Systems, devices, and methods for controlling access to vehicles in rental, loaner, shared-use, and other vehicle fleets. Some of the present systems, devices, and methods use encrypted virtual keys that can be relayed to a vehicle computing device via a user's mobile device. Such virtual keys can be command-specific such that successful use of a virtual key results execution of a predetermined command or group of commands, and further commands require one or more additional virtual keys with the additional commands. Others of the present systems, devices, and methods provide tools: for provisioning or initial pairing of vehicle computing devices with corresponding vehicles, identifying and permitting a user to select locally available vehicles, prompting vehicle computing devices to retrieve pending commands from a server, and/or various other functions described in this disclosure.