I wondered whether laughing gas was somehow current, but the dates I could find place it earlier (mind you, I didn't go beyond Wikipedia, so there may be something in this...)

She has significant sartorial vanity, but she refuses to say "I did this."

The vanity certainly includes the sartorial, but I think goes beyond it. Laughing Gas opens with her admiring herself in the Tobacconist's window, which reflects her in triplicate: "She thought it was such a lovely sight that she wished there had been a dozen of her or even thirty. The more Mary Poppinses the better."

The refusal to say "I did this" isn't modesty, though: her vanity isn't about what she does, but what she is.