Its the pun (though NOT IMO the "worst" sort of pun).. jut the pun on vices in itself..

But teh general tone of her conversation just then is enough to make Edmund grave. Seh is rude about the Navy, and the lower ranks to which Fanny's brother belongs. She makes it clear that she does not like her uncle.. who brought her up... She is clearly misjudging her company and showing a lack of ability to address herself to the company she's in....Her slightly racy discourse might go down wlel enough in London but its not suited to a country house and the inhabitant of a parsonage....