Dimitri Vasilevitch was a member of Earthforce and the second-in-command of General Yuri Mikhailovich Denisov[1]
Behind the scenes
It is possible that his name was in fact Dimitri Vasilev, and Vasilevitch is just another instance of the novelization mixing up Russian last name and patronymic, as it calls Yuri Mikhailovich Denisov Yuri Mikhail Denisovitch.