Pfizer expects the vaccine to be good for up to one week at -8°C to +2°C, so the difference in stability or storage life at -70°C or -80°C might be a small issue compared to what difference it makes on the requirements on the continuous cold chain. It sounds like using dry ice might be a 90/10 solution, as Michael Seibel would call it: get 90% of the result with 10% of the effort.