If its found to be mareks I would definitely vaccinate and give those vaccinated birds a good amount of time to develop immunities and antibodies BEFORE letting them outside. I've read it takes a good 14 days for antibodies some say even longer so keep that in mind when vaccinating.i wish you the best.
I've been feeding her weetbix with honey and egg yolk, when the chemist opens I'm going to get b vitamins and st jons wort, as I've seen it mentioned as a "cure" for mareks, although the claim seemed unsubstantiated. My chickens all free range in my backyard, but we have only been here since Tuesday, is the mareks likely to come from our previous home, or from the new home?