Just to make a concise post, and because it was a pain to find: on the 1st Gen AWD Vibe, the downstream o2 sensor comes up directly beneath the passenger seat, and plugs into a connector beneath the seat as well, under the carpet.
You are best off removing the passenger seat, as well as the airbag computer bracket, and the door rail plastic to be able to pull up the carpet enough to access both the connector point , and the hole. I cut the carpet on mine a little to help things along.