I doubt Vermont would go through with it. If they did, Britain would accept at once, since it would be a huge boon as far as pride is concerned ("rebels seeing the error of their ways, graciously welcomed back-- look at how noble we are, welcoming them as brothers, and look at how impotently those other upstarts rage at it").
If it did happen, the USA isn't really in a position to successfully invade. Under these circumstances, Vermont isn't a state trying to throw off the British, and attempts to capture it will meet the same fate as attempts to capture Canada. Indeed, the USA will be "1812'd" if it tries. By which I mean: they'll get their asses kicked, and after that it's
status quo ante. (Because the British war goal here -- keep the USA from conquering Vermont -- would be
substantially more attainable than "keep the thirteen colonies under our control against their will".)
ETA: Also, to underscore
@TC9078's point, Napoleon won't be a factor
at all, in this scenario. These events take place prior to 1791 (Vermont's OTL entry into the Union). The French Revolution is a thing, but Britain's commitment to that matter is still far more limited at this stage than its commitment to the Napoleonic wars in 1812. Long story short: if Vermont decides to go through with this, the USA is best served by just accepting it. Attempts to force the matter won't end well for the USA.