You may be able to reduce the size if you have additional vent pipes through the roof for other sinks, toilets, etc.: I have removed a section of the dry-wall in a 2nd floor bathroom to install a medicine cabinate. There is a 3 PVC pipe for venting the plumbing directly where I planned to install the cabinate. The cabinate is 34 high and 10 inches wide. I wish to move this pipe around the cabinate. The 3 pipe is tight against both walls not leaving much space to wrap a flexible pipe w/ PVC cement. Is there a special flexible pipe for this purpose? If not is there any issues with taking the pipe down to say 2 and then running the flex pipe around the cabinate and reattaching to the pipe above?