Jim, I agree with you that vaccinated chicks do not transmit virus. They are vaccinated with a strain of Turkey virus which elicits an immune response in the chicks. Turkey virus does not replicate in the chicks and they are not capable of shedding or transmitting it to other birds. There seems to be a lot of rumor going around lately that says otherwise, but turkey virus does not grow in chickens.
The only thing I can assume is that the hens you had shipped from Texas were never exposed to any kind of Marek's virus and therefore never developed any immunity to it before you got them. It is not common, but it is possible. Apparently the virus thrives in damp environments, so perhaps the farm in Texas was very dry and they never had Maren's there. Are you positive they died from Marek's, or could it have been something else?
Personally, I keep my chicks isolated for 6 weeks after vaccinating them. That said, I still had one die at 14 weeks. I also had two chicks develop signs of Marek's and then fully recover! After having a 95% death rate in one hatch I don't believe in letting the "weak" ones die off and only letting the resistant ones survive. If I did that I would have almost no chickens left.
My Marek's probably came from one bird I bought from a back yard breeder in Seattle. I still have the hen and she's never been sick. He called me after I'd had her for about 8 months to tell me he wasn't selling chicks anymore because he though he had some die from Marek's.