Vent stacks only need to go as high as the average snow fall for your area dictates. They do not have to go above the roof line. They must NEVER be capped.
I would agree that what you smell is sewer gas, however, it may simply be the result of a cracked vent pipe or dislocated joint on the vent stack.
It is also possible that you do not have a proper trap on the main sewer line to your house and are therefore feeding direct sewer gases up thru the roof vents. This would cause sewer odour under proper wind conditions.
Pouring bleach or anything into the stack won't help.
You should probably have a pumbing professional come by and run a video camera thru your sewer lines and vents to determine if and where any break in the line may be occuring.