Last weekend, the MIL light came on in my 2003 3.0 Quattro sedan. I toook it in to the local Audi dealership and they hooked up the test equipment. They got a code indicating a vacuum leak. They replaced one of the vacuum lines. Drove away happy but $105 poorer. The next day the light was back on. The car feels fine, no indications of any performance loss. I called Audi and they said bring it in again and they would test it again. I don't want to spend any more money chasing phantom issues. Has anyone had similar experiences and how did you resolve them?

Well if they said the cause of the MIL was a vacuum leak and you paid for that, they better replace that for free again. Also, ask them to get a code-it will tell u where the problem is.
Also, if they will ask you to pay their "favorite" fee of like $100 just to diagnose it, dont pay it, they gotta scan it for free-cause it is the second time and they didnt fix the problem the first time. Good Luck.