Great question! 

In the troposphere, there is a region of high pressure sitting over the Arctic/Greenland...it's very negative phase of the Arctic Oscillation (https://www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/ao…). In this case, the reference to the polar vortex may correspond to the tropospheric jet stream rather than the stratospheric polar vortex.  

But that tropospheric circulation pattern is likely the immediate reason for the colder air coming. The question is whether there are other factors that are causing this pattern besides normal fluctuations in the atmosphere. It seems like both the La Nina and the Madden Julian Oscillation could playing a role.

As to the role of the stratosphere (and the polar vortex), that is less clear. The polar vortex is still quite strong and mostly over the polar region, but stretched out over North America and Asia. The strength of the polar vortex is not coupling down to the troposphere, but  the stretching does seem to be more consistent with the troposphere. So it's possible that the stretching of the stratosphere is nudging the tropospheric jet a little further south. All that to say there are many factors that could be impacting the cold air outbreaks and the stratosphere's role is probably not a major player here.