Article IV, US Constitution:
"Section 3. New states may be admitted by the Congress into this union; but no new states shall be formed or erected within the jurisdiction of any other state; nor any state be formed by the junction of two or more states, or parts of states, without the consent of the legislatures of the states concerned as well as of the Congress."
Why would Vermont consent to the loss of a producer of tax income?
No new State is being formed, one part of a State is simply joining another state. It is not as though Killington wants to join Hanover, NH and create a new state.