Talking isn't the same as doing. And no, secession wasn't an unknown theory - it had been around at least as long as the Kentucky-Virginia Resolutions. But it is one thing to theorize about a thing and another to actually take action and test it. The Hartford Convention did not advocate disunion and in fact disavowed it. The Hartford Convention did not pursue secession and even the specter of it raised condemnations of treason from Republican-Democrats throughout the country that brought down the Whig party.

As for Vermont, she did not secede from the Great Britain; she rebelled, declared her independence and ultimately won it through victory in the revolution.

BTW, what is the source for your quote?