MDV is super easy to transfer from one flock to another, so if both sets of birds are on the same premises, then you’ve probably already inoculated the chicks if you do have MDV in your older birds. That’s why vaccination is supposed to happen in the egg or at 1 day old, because the virus is considered to be ubiquitous in poultry keeping environments, and the vaccine only works if it happens before natural infection. If your older, vaccinated birds don’t have wild MDV, then they won’t infect your naive chicks with disease-causing MDV anyway, regardless of where they are housed.