What's the difference between the media manager expiration date (given by vmquery) and NetBackup's expiration date (given by bpmedialist)?

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The expiration date given by vmquery and other media manager commands is the date the tape ITSELF should expire (i.e. get thrown away). This is sometimes referred to as its _physical_ expiration date.

The expiration date give by the bp* commands is the date the data on the tape expires (i.e. can get overwritten).

I personally do not use the MEDIA expiration date for anything. In normal use, date has little to do with a tape's usefulness. What matters is the number of tape passes. Even so, most people never get anywhere near the useful life of a tape before they move on to some other type of media. Most modern tapes are rated at over 30,000 passes. Even at 10 passes a day, that tape would last your almost 9 years. However, most tapes don't get 10 passes in a month. They get written to and then set on a shelf in the library for quite a while. (I will state that I am just one person, and their are people that disagree with the above opinion. That's OK. They're wrong.) ;)