You've already accepted, but to properly fix this:
I'll assume it's all polyethylene pipe (typically used for well piping). The fitting also looks like nylon, which in my experience is easy to damage and often leaks. I much prefer PVC (which is typically gray) or brass.
Take everything apart, remove the elbow, and throw it away. Heat the end of the poly pipe with a torch, both around the outside, and a tiny bit on the inside. You want it to be warm, but not melted/squishy/burning. Put on two clamps, then put the fitting in, and then tighten down the clamps (about 1/4" apart, give or take). Repeat with the other end.
If you've never done this, you may want to practice on some scrap pipe first, especially if you don't have a lot of extra to work with. If you do melt the pipe, you need to cut the end back and remove the melted piece.
IANAP (I Am Not A Plumber) but I have recently replaced my old cast iron stack.
The relevant part of the International Plumbing Code looks like section 904. Specifically the following sections may apply to you.
904.1 Roof extension. All open vent pipes that extend through a roof shall be terminated at least [NUMBER] inches (mm) above the roof,
except that where a roof is to be used for any purpose other than
weather protection, the vent extensions shall be run at least 7 feet
(2134 mm) above the roof.
Note the [number] is usually specified by your local codes as it is very specific to each region. Some web searching turns up a few forum posts from plumbers in Ontario who say it needs to be 6 inches above the roof. That does not sound like much to me and if you are already at 6" and having issues it seems reasonable to go higher.
904.2 Frost closure. Where the 97.5-percent value for outside design temperature is 0°F (-18°C) or less, every vent extension through a
roof or wall shall be a minimum of 3 inches (76 mm) in diameter. Any
increase in the size of the vent shall be made inside the structure a
minimum of 1 foot (305 mm) below the roof or inside the wall.
904.3 Flashings. The juncture of each vent pipe with the roof line shall be made water tight by an approved flashing.
904.7 Extension outside a structure. In climates where the
97.5-percent value for outside design temperature is less than 0°F (-18°C), vent pipes installed on the exterior of the structure shall
be protected against freezing by insulation, heat or both.
I don't know the preferred method for doing this but if you plan is to add a coupling from inside the house and "push" the pipe up, I bet you will compromise the water tight seal around the pipe at the roof. The simplest solution would be to add this extension at the very top so you don't have to mess with the flashing at all.
If this is the main waste/vent stack for the home, I don't think you are allowed to reduce the diameter of the pipe (even up at the "vent" portion). I don't know if that is still true for smaller vents that service just a few fixtures.
If it is black plastic, it is ABS and should be easy to cut and glue.
Best Answer
According to the marking on the tube it's rated for both EN14680 (Adhesives for non-pressure thermoplastics piping systems) as well as EN14814 (Adhesives for thermoplastic piping systems for fluids under pressure).
The product datasheet I found says:
But - on page 2 it says:
My best guess is that it'll probably work sorta-kinda-ok-ish, but I wouldn't use your PVC-U on ABS pipes except as a last resort in a non-demanding application.