I'm stuck with the same problem regarding AppTransaction and volume purchase program customers.
Did you find a solution?
It looks like it would be possible to use the AppReceiptValidator library to locally determine if the environment is "ProductionVPP" (see https://github.com/IdeasOnCanvas/AppReceiptValidator/blob/49e95b1d67ba5c86b016231264832e12496897e8/Sources/AppReceiptValidator/UnofficialReceipt.swift#L32).
It's impossible for me to test this though since I don't have access to a sample VPP receipt. I'm not sure if a VPP receipt does contain the originalAppVersion field for example.