Why would he have to go and get Vlad's body? Nagash animated an entire empire before now and he never even left his fortress at Nagashizzar to do it. From what we're hearing he has been brought back at full strength so raising one vampire lord long distance hardly seems difficult by comparison and once awake I'm pretty sure Vlad could break out of his tomb and make his own way back to Sylvania, what with vampires being able to turn into mist, bats, wolves etc.