That's because most industrial vacuum processes happen in the 15 psi context of conditions on Earth. We already know that the Sabatier process will work in Mars like conditions. The chemistry has been around for a century, and demonstration hardware specifically for Mars conditions was built over a decade ago.
This thread seems to suggest that the big barrier is energy. The reason why natural gas and petroleum are used as feedstocks for the manufacture of plastic on earth is precisely because using them reduces the required energy input. The thread is also a good source of a few interesting links.
In terms of having enough output from a material standpoint, the requirement is still mostly energy. It would require a lot of energy to move enough CO2 through the plant.