] General Tilney makes Catherine leave on a Sunday. In `Persuasion', written much later, Sunday travelling appears as something of a vice, but nothing is made of it here.

From your subject line I thought you meant this:
“Good-bye. As tomorrow is Sunday, Eleanor, I shall not return” (Henry when he leaves to scare his housekeeper out of her wits to prepare dinner).
That means that Sunday traveling IS a vice here, too, at least in Henry’s opinion.