Vermont was never officially recognized by anyone as free & independent of the United States.
Throughout our history states & territories have formed or changed status according to Constitutionally mandated "mutual consent".
State boundaries have been changed, states have been split into two or more, proposed states absorbed (i.e., Franklin, Jefferson, Deseret) by others, territories added & "retroceded" -- i.e., Philippines, Panama Canal. All that was perfectly normal & constitutional.
So, land which came to be called first "New Connecticut" and then "Vermont" was previously claimed by New Hampshire, Connecticut & New York.
The issue was settled by Congress admitting Vermont as a free state, then Kentucky as a new slave-state, the first such pairing among many.
Mutual consent for such things is what the Constitution requires.
Vermont was never officially recognized by anyone as free & independent of the United States.
Vermont v. New Hampshire, 289 U.S. 593 (1933)
At 596:
Vermont was admitted as a free and independent state."
Only a free and independent state can be admitted as a free and independent state.
At 608:
... her independence was recognized by New Hampshire in 1777, by Massachusetts in 1781, and by New York in 1790."
Vermont was a free and independent state from 1777 to 1791. Her independence was recognized by NH (1777), MA (1781), and NY (1790). Vermont acted as a free and independent state for fourteen years. During this time, Vermont was not a state of the United States, nor was Vermont a territory of the United States. Vermont was self-governed, totally independently. She coined her own money and had her own postal system.
Vermont entered the Union in 1791 as a free and independent state. You can't get more recognition of being a free and independent state than a holding of the U.S. Supreme court.
Mutual consent for such things is what the Constitution requires.
Vermont was explicitly NOT admitted under Clause 2 of Article 4, Sec 3. You keep prattling on as if Clause 2 applied. It did not. Vermont was admitted and a free and independent state which achieved that status by their successful revolution of 1777. Vermont was explicitly found to have been admitted under clause 1.