Yes, it is possible to 90 a plumbing vent pipe. The steps required are:
• Measure the existing pipe and cut the new piece of piping to match.
• Remove any burrs from both ends of the piping with sandpaper or a file.
• Place a rubber sleeve on one end of the pipe and slide it over until it meets at the 90 degree angle mark.
• Apply adhesive or sealant around both sides of the elbow joint before pushing them together firmly and evenly.
• Secure with clamps if necessary, then let dry completely before testing for leaks. With these simple steps, you can successfully create a 90-degree bend in your plumbing vent pipe!