The vaccine, ultimately, prevents the symptoms only but not the actual infection. So basically, if a vaccinated chicken is exposed to Mareks it will catch the disease but not die. If an unvaccinated chicken is exposed to Mareks it will catch the disease and probably die.
If an unvaccinated chicken is exposed to a vaccinated chicken, whether or not the unvaccined bird dies depends on whether or not the vaccinated bird has been exposed to Mareks which you have no way of knowing because there's no symptoms.
Vaccinated chickens who have been exposed to Mareks can basically become Typhoid Mary's - carriers that appear healthy but get every unvaccinated bird around them sick. This is why the advice of not to keep a mixed flock (vaccinated-unvaccinated) exists. If you bring vaccinated birds that seem healthy into an unvaccinated flock they may be carriers and get those unvaccinated birds sick.
On the other hand, if you have a vaccinated chicken that has never been exposed to Mareks it's perfectly safe to mix with unvaccinated chickens. But how do you know it's never been exposed to Mareks? You often just don't. So many people treat flocks where all the chickens are vaccinated as mareks positive because you just can't tell.
Now as to whether or not YOU should vaccinate... That depends largely on WHY you're keeping chickens.
For example; If your chickens are pets, their lives may matter deeply to you especially emotionally and so you may want to vaccinate them because that way if they DO get sick they have a real chance of living.
If any of your chickens are diagnosed with Mareks it's probably a good idea to vaccinate, that way your chickens have a chance of living because without it a lot of your birds would die.
If your are a closed flock where birds will never leave you may want to vaccinate just to increase the chances of keeping your flock alive generally.
If your chickens are NOT pets or your flock is Mareks free, you may NOT want to vaccinate because you may want to be aware of if or when Mareks hits your flock. If you, say, breed chickens to sell chicks, pullets or hatching eggs having a Mareks free flock is critically important. Knowingly spreading undisclosed virulent deadly livestock diseases can even be a personal liability that gets you sued. So keeping your flock vaccine free might mean they all die someday (maybe) but it might also save you a lot of risk, money and give your chickens a selling point.
Additionally, as said in the great big Mareks FAQ, many of the vaccines are losing their ability to produce a strong enough immune response as the disease mutates. As it is, most commercial producers consider the Mareks vaccine to be 90% effective at keeping Mareks infected birds symptom free. The other 10% (in commercial flocks) still develop serious symptoms and die from it. That percentage of protected birds has dropped awfully low for some vaccine strains as the virus mutates.
And the prevalence of vaccine use has already bred hotter strains - there's a real chance of the disease continuing to get worse and the vaccines continuing to be less effective as people continue to use the vaccines. Does the vaccination of your particular flock of just a few birds influence that? Probably not but maybe,. And we KNOW the more people using them the worse the disease is likely to get world-wide.
So in deciding whether or not you, in particular, should vaccinate you should probably ask yourself;
What are my goals for chicken keeping?
Am I going to be distributing birds to people who need to know if I have Mareks in my flock?
Am I willing to risk my chickens dying to know if they get sick from Mareks?
Do I think my use of the vaccine will influence the world for better or worse? Or does it matter at all because I'm just one little flock?
And then based on those answers and armed with the knowledge that the vaccine keeps birds alive but doesn't stop the spread of the disease itself you can make an informed choice.
Are you keeping a few pet birds with good bio security? Vaccinate.
Are you keeping a breeding flock that's important to KNOW is Mareks free? Don't vaccinate.
Something inbetween? Look at your factors and decide for yourself.
