1. A virtual machine control method for a computer on which a plurality of operating systems are available to be run on virtual machines under control of a virtual machine monitor running on the computer, said computer having scalar and vector processors wherein the vector processor starts executing vector processor instructions in accordance with an execution instruction which the scalar processor sends to the vector processor in response to an execute vector processor instruction issued by an operating system whereby a virtual machine to which the operating system is assigned uses the vector processor, said method executed by the virtual machine monitor comprising the steps of: selecting one of the operating systems for running on the computer; determining whether any of the virtual machines other than a virtual machine to which the selected operating system is assigned is using the vector processor; suspending the running of the selected operating system on the scalar processor when it is determined that one of the other virtual machines is using the vector processor; and dispatching the scalar processor for exclusive use by the selected operating system when it is determined that none of the other virtual machines is using the vector processor.