Toilet Refuses to Flush
The lack of venting does cause the drains to flow slowly. In your case, mostlikely caused by the stack freezing again.

I would suggest you do one or a combination of the following:

1) Insulate the exterior portion of the vent stack. That way it is less likely to freeze during the coldest time of the year (one time fix)

2) Add a heating element around the top of the vent stack. I did a quick search and found a product called ArcticVent. This keeps the exterior and upper portion of the stack warm so that it does not freeze. (one time fix)

3) Pour deicer down the vent stack when it does freeze. Deicer would melt the ice. Obviously this would be a quick fix and you wouldn't want to have to do this often, especially if there is snow on the roof. (repeated and temporary fix)