I’m assuming that you’re talking about a high efficiency furnace up in your attic. The problem with connecting condensate discharge pipe directly into your plumbing vent stack is not only the code issue nor the amount of condensation / water dripping into the vent – it is about the bacteria from the sewer system vent pipe that could travel through the condensate discharge pipe and contaminate your heating system.
This becomes even more critical if you also have an AC coil installed in the attic. In order to discharge condensate into the plumbing vent stack, you have to cut it (the vent stack pipe) and install a p-trap that would allow for an indirect connection and prevent contamination with bacteria. Another benefit of this connection is that you’ll be able to easily spot a problem / clogged condensate line just by checking it periodically.
If you still have a problem with lines freezing, cover the condensate discharge pipe and the trap with insulation or (more expensive solution) build walls around the furnace and discharge area – this would slightly elevate the temperature and prevent condensate from freezing.
Let me know if this helped or if you have any other questions.