As dances were (and still are) a common way of meeting a future marriage partner the context is irrelevant. St John Visnney was warning her about getting involved with someone with bad intentions. As well as reminding her of the value of the scapular when worn. The fact the meeting was at a dance is irrelevant.
And your interpretation is also speculation, as if you can know this for certainty. You're reading into it what you want to see here. Before I saw this I wrote that this COULD BE a possible interpretation, but it also COULD BE that he considered it sinful. While the author of this anecdote concluded that he considered it sinful per se, it COULD be something else that motivated him here. There are other incidents I've read about in the past, however, which indicated that he did consider it sinful per se. Now, that author did make a point that St. John Vianney strongly implied that SOMETHING sinful took place there, since in asking if she had forgotten something, this suggests there was a defect in the matter for the Confession, i.e. an omitted sin, presumably mortal? ... since there's no strict requirement to confess venial sins, although perhaps he was being more thorough with this person. We just don't know is the point, and we don't have enough information, either for the author's conclusion, OR for yous.
You should honestly examine your conscience, about whether you have ulterior motivation for constantly advocating on behalf of the liceity of dancing. It's not that one could not objectively come to the conclusion that certain forms of dancing are licit and moral, just that it's clear to us that you have some kind of ulterior motivation tending toward a certain conclusion that you want to be the case up front, and that kind of faulty emotion-driven begging-the-question reasoning can lead to false conclusions.