As I understand, Zcash shielded and deshielded transactions use a JoinSplit when converting notes from shielded to transparent (and vice versa).
vpub_old - is the value that the JoinSplit transfer removes from the transparent value pool, and
vpub_new - is the value that the JoinSplit transfer inserts into the transparent value pool, signify how much is being transferred.
vpub_new both simultaneosly have values in the same Zcash transaction? If so, is there an example transaction in practise?