[SedVictaCatoni]

In order for this town to seceed, both the Vermont and New Hampshire legilatures would have to approve it as well as the US Congress

But Killington isn't trying to become a state. This section governs only the procedure by which new states could be created by breaking up or merging old states.

[Paleo Conservative]

This section governs only the procedure by which new states could be created by breaking up or merging old states.

Well, New Hampshire would be re-formed with parts of another state. I think that is covered by this clause.

[Paleo Conservative]

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.

As I recall from logic a set is a subset of itself. The whole state of New Hampshire is a part of New Hamphire. Taking part of Vermont and adding it to New Hampshire would be forming a new state from the junction parts of states.