Sorry, this is not true.
Marek's vaccine is made from Turkey Marek's. It can not cause disease and/or carriers of chicken Marek's. You can have a mixed flock of vaccinated and nonvaccinated birds with absolutely no problems at all.
As for the protection provided by the vaccine. The vaccine is only 90% effective and only prevents the tumors associated with the disease from forming. It does not protect chickens from being infected or from becoming carriers of the illness. The only way that a carrier is formed is if the chickens are exposed to a bird with Marek's and survives. Again, the vaccine CANNOT cause Chicken Marek's or produce carriers of Chicken Marek's. It is made from Turkey Marek's virus and it is impossible for that to happen!