What might cause audi a4 b7 vacuum pump oil leak?

The most common reason for an AudiA4 B7 to spring an oil leak from the vacuum pump is a failure of the vacuum pump o-ring. This is a very common issue on the B7 generation, though Audi has never put out a technical service bulletin on the matter. Alternatively, it's possible that the entire pump has gone bad, and has sprung a leak that way; this will, unfortunately, necessitate replacement. Many drivers report replacing the entire vacuum pump when the issue arises -- while this is at Your discretion, I'd recommend trying the o-ring method first, as the difference is several hundred dollars. Unfortunately though, if this doesn't fix Your issue, You'll still be out the hundreds of dollars, and just a bit more work.