The polar vortex is present already and is slowly deepening. This is clearly evident in 10 and 30 mb height and wind charts.
Meteorological fall begins September 1. As noted, the polar vortex begins developing in August (because of the decreasing sunlight post-solstice) and isn’t full strength until later in the season. At this altitude in the poles (30 km up) thermal lag is less pronounced. Note the American Meteorological Society glossary says “the stratospheric polar vortex exists from fall to spring” (source). It is not just a winter phenomenon.
There is some predictive value in the strength of the polar vortex, especially with regard to the jet stream and sudden stratospheric warming events, but all seasonal forecasts carry uncertainty.
I am a meteorologist in Canada working primarily in the energy industry and have successfully used the polar vortex (among other parameters) in medium- and long-range forecasting several times. This has generally been in the context of deep troughs bringing heavy snow to Western Canada and the U.S. Northwest.
Of course, this being the internet readers can take or leave that claim. It was especially useful in early forecasts regarding an extreme cold weather outbreak in the U.S. Northwest in February-March 2019.
I think La Niña plays a bigger role for the whole season than this. Where I am (Vancouver, Canada), I expect below average temperatures and above average precipitation. The presence of a weak polar vortex signals a higher risk of at least one strong winter cold snap in North America, but we won’t know specifics until much further along in the season.