I don't know a huge amount about Marek's, but I have done a lot of reading about the subject recently, and one thing I am pretty certain about is that a vaccination will help to protect the bird that is injected, but does not pass any immunity on to chicks.
If you want to vaccinate your chicks then it is worth knowing that you can only buy the vaccine in bottles of 100 doses, and that once activated for use the vaccine has to be used up within a couple of hours. If you are vaccinating a broody hatch then you will be throwing away the majority of the vaccine, but you may still consider it worth doing in order to provide immunity to your birds.
I am not certain about taking the chicks away from the broody hen to quarantine them - that seems rather cruel, as she will be in full mothering mode, and will not understand what has happened to her chicks. Marek's is spread through chicken dander, and so if your chicks hatch under the broody they will almost immediately be exposed. In addition, unless you practice extreme bio-security on your property you can be certain that there is chicken dander pretty much everywhere already - either blown by the wind or carried around on your clothes / shoes etc. so quarantine may not be as useful as you think. I am considering vaccinating my broody hatched chicks this summer, but I shall leave them with their mother and let nature take its course.