help! dealer rape!

[kehale]

I have an 01 1.8T. The dealer wants to charge me $340 to replace an Oxygen sensor...here's the story.

The dealer recently fixed an oil leak (60,000 miles)
When I got it back, it didn't leak oil, but...

when it was warmed up, it made a hissing sound at idle (like a leak in an air hose). It wasn't terribly loud, but when you opened the hood you could hear it quite loudly.
Also, when you got on the gas (basically under boost conditions), you could hear a rushing of air (like the wastegate needed a muffler.
I know these things run on 7psi of boost and bleed off a lot, but...it didn't sound like it used to (almost embarrassed to drive it)

I was going to take it in when it needed an oil change, but the other day, the "check engine" light came on.
I dropped it off and now they're telling me that I have two vacuum leaks and the O2 sensor is bad.

Can the vacuum leak kill the o2 sensor? (or some other leak contaminate it?)

I think it should the 02 sensor should be under warranty (especially if it was caused by a warranty problem).

Your thoughts?
-T

PS: this car is totally stock and just my daily driver.