Oil Usage Problems in 02 Corolla

I have a 2002 Corolla w/ 77K miles on it. It is using up oil fairly quickly between oil changes. After having it checked at a nearby garage, I was told there are no signs of leakage or of burning oil (the only problem was a rattling noise and finally the oil light came on once and went off - both problems ceased after adding oil). They said to drive it 500 miles and bring it back. I checked the dipstick after hitting 500 miles and there was no oil on it. I added oil and haven't driven it since, but the level on the dipstick has not decreased.

I have heard that, worst case scenario, the car could need a whole new engine. I have also been told that a plugged up vacuum valve has caused such symptoms in Toyotas. Any thoughts?