Wow, that's a loaded question..I will take them one at a time.
Yes, you can find the vaccine for after the chick arrives. But, you have to buy it in a 1000 doses. Several places have it for sale. It must be kept refrigerated and you do it yourself, at home.
Yes, your bantam could get Marek's and the rest of your flock would be at risk. There is a low chance of this if you do not bring in any other birds. If you are planning on not getting any other birds or just ordering them from the hatcheries then, I would say that you would be safe without having this little guy vaccinated.
The vaccine is only 90% effective and only protects the birds from developing the tumors associated with the disease. It does not prevent them from getting Marek's or from becoming carriers.
If you have any other questions, feel free to ask.