Ditto what Kerry said. September is variable. First week or two often hot -- still summer weather, temps in the high 20s or 30s (70s or 80s) in the daytime. Evenings a bit cooler.
Late September can be cooler and rainy at times. It all depends.
Our WritersFest is the end of September. Kingston is a bit cooler than TO with a slightly earlier autumn. I have worn everything from a sleeveless dress without hose to long sleeves, a jacket, and tights with booties to events at this festival (in different years and over the course of that week).
I would say: if the Vince dress works on you, it will be perfect. A sleeveless dress will work great if you have a topper for it just in case. Jeans will work in the evenings. Lightweight wool at night but we are not talking cashmere weather. You may even want shorts in the daytime for casual. I was definitely wearing shorts well into September last year. And sandals.
