Can an employer stop you from smoking in your private vehicle at break time or lunch time, i live in nys.?:
Are you sure that you are accurately reporting the information the attorney gave to you?  If your vehicle is parked on the private property of your employer, the laws of your state may very well give them the right to enforce this policy.

Regardless of what you feel about the policy, chances are that you are an "at will" employ unless you belong to a union.  I am not aware of any state or federal law that would protect you if your employer fired you for violating the policy.  So you have two choices:  (1) stop smoking; or (2) go off the property to smoke.