Sounds like your on the right track, checking the basics like vacuum hoses.
If you have a wide band air/fuel sensor reading after the cat, and your cat isn't hollowed out, its going to give false air/fuel readings. To properly read what and how your engine is burning, the sensor needs to be before the cat. The pre-cat sensor and a post-cat sensor on modern cars have two different and separate functions. The post-cat sensor is only there to tell the computer that the cat is working- it has no relation to the air/fuel trim. Verses the pre-cat sensor that tells the ECU how to "fine tune" the fuel trim by reading the left over fuel coming out the exhaust manifold. That would explain why the sensor is reading so lean- if your cat is working it should show a lean situation after the cat, since the cat turns exhaust fumes into CO2 and H2O. Also when you rev the car, it pushes more fuel down the piping so you should see an increase of your A/F ratio until the cat catches up to the increased fumes,
A trick is to get a "bung" welded in before your cat, probably close to your other primary O2 sensor at the collector. The important thing is that the sensor bung is on the upper half of the collector- other wise if its too horizontal, it can get moisture and such in it.
