I am not sure that Emma would be so unfair to the industrious of the area when she is not working on her own agenda (like turning Harriet's attention from Mr. Martin). In ch. 10, Author Austen takes the trouble to point out one of Miss Woodhouse's virtues, in unalloyed praise for once:

Emma was very compassionate; and the distresses of the poor were as sure of relief from her personal attention and kindness, her counsel and her patience, as from her purse. She understood their ways, could allow for their ignorance and their temptations, had no romantic expectations of extraordinary virtue from those, for whom education had done so little; entered into their troubles with ready sympathy, and always gave her assistance with as much intelligence as good-will.

However, maybe it was not her schemes for Harriet that caused her to dismiss them. I have seen many posts that think Emma must have crossed the bounds of decorum, pressing Jane Fairfax that first evening at Hartfield, so perhaps we can safely say that the Misses Cox simply gossiping about Mr. Martin and asking Harriet meaningfully about him was enough to class them as 'vulgar.'