If you do vaccinate the birds they in turn will be carriers so you will need to do the entire flock. Any birds you bring in will also need to be vaccinated.
One thought about the birds that you lost is that the disease could have been contracted almost anywhere. It could have been as simple as following in another's steps at a stop. If that person had infected droppings or soil on their shoes and then it transfered to your own it could have been spread that way.
This is a quote from the Merck Vet Manuel.
Chickens are the most important natural host for Marek's disease virus, a highly cell-associated but readily transmitted alphaherpesvirus with lymphotropic properties of gammaherpesviruses. Quail can be naturally infected, and turkeys can be infected experimentally. However, severe clinical outbreaks of Marek's disease in commercial turkey flocks, with mortality from tumors reaching 40%80% between 8 and 17 wk of age, were reported in France, Israel, and Germany. In some of these cases, the affected turkey flocks were raised in proximity to broilers. Turkeys are also commonly infected with turkey herpesvirus (HVT), an avirulent strain related to Marek's disease virus that is commonly used as a Marek's disease vaccine in chickens. Other birds and mammals appear to be refractory to the disease or infection.
Marek's disease is one of the most ubiquitous avian infections; it is identified in chicken flocks worldwide. Every flock, except for those maintained under strict pathogen-free conditions, is presumed to be infected. Although clinical disease is not always apparent in infected flocks, a subclinical decrease in growth rate and egg production may be economically important
And this is about transmission of the disease.
The disease is highly contagious and readily transmitted among chickens. The virus matures into a fully infective, enveloped form in the epithelium of the feather follicle, from which it is released into the environment. It may survive for months in poultry house litter or dust. Dust or dander from infected chickens is particularly effective in transmission. Once the virus is introduced into a chicken flock, regardless of vaccination status, infection spreads quickly from bird to bird. Infected chickens continue to be carriers for long periods and act as sources of infectious virus. Shedding of infectious virus can be reduced, but not prevented, by prior vaccination. Unlike virulent strains of Marek's disease virus, which are highly contagious, turkey herpesvirus is not readily transmissible among chickens (although it is easily transmitted among turkeys, its natural host). Attenuated Marek's disease virus strains vary greatly in their transmissibility among chickens; the most highly attenuated are not transmitted. Marek's disease virus is not vertically transmitted