The reasons for the freeze up of the vent has many reasons and for most of these you can do very little. The 45° cut on the stack at the roof may help a little but I doubt it.
Different plumbing setups with different conditions can cause yours to freeze whereas your neighbors does not.
Pouring hot water down the stack from the top may thaw the clog but it's risky being on the roof in winter and it won't prevent it from reoccurring.
Heating the pipe is an option even though you don't have attic access because you can heat it from the inside with a narrow heating cord. I saw this described on a site at
Frozen Vent Stack and Water Lines