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The polar vortex nearly dies every year around springtime in the hemisphere you are concerned with. This failure is what brings those spring frosts in February, March.

It's not really possible to break the vortex by adjusting mean tenperature.

What creates the vortex in the first place is temperature gradients. 

Starting in fall, the pole you care about, and everything above the arctic circle, doesn't see sunlight for six months. Temperatures drop (relative to parts of the Earth that are enjoyinv solar heating). The temperature drop causes a pressure drop (PV = rho R T) (again, pressure relative to the rest of the  and you get a forming and strengthening cyclone holding the coldest air within.

When the sun rises in local spring, the rising temperature at the pole breaks that low pressure grip, and that very cold air breaks out towards the temperate and tropic regions.

Change to the average temperature of all these events, don't change the dynamics: cooling will happen at the poles, it will drop the temperature, which will create a cyclone.

But let's say something breaks it forever. The cold air will empty out as it does every spring season, but it will be exceptionally severe as in 1935 or 1949. Maybe lasting a few weeks, instead of a few days. 

But then, the reservoir of super cold air will be empty, and unless something allows a cyclone to form seasonally trapping the cold air geographically, then future winters will be milder as the cold air is allowed to mix with tropical and temperate air during winter.