Following test was performed to check if PXC 5.7 can work with Keyring vault plugin.

Below error is seen in the logs:

If same steps are performed on PXC-8.0, there are no errors.

Also, if we use keyring_plugin instead there are no failures on both PXC-5.7 and PXC-8.0

A line by line comparison of logs was done when node 2 is started with keyring_file plugin and keyring_vault plugin.

Logs:
node2 logs keyring_file (success): https://gist.github.com/mohitj1988/a5cfe7b4d9478f0c2667efb0c1266a49

node 2 logs keyring_vault(failure):
https://gist.github.com/mohitj1988/236a0d78609992e7bc9a0048713c259e

innobackup.prepare.log: https://gist.github.com/mohitj1988/80da4841610933179a2184fa432a814a

I have also attached 2 scripts to repeat the issue.

Script 1: Perform tests on PXC-8.0 using keyring_vault plugin (Success)
Script 2: Perform tests on PXC-5.7 using keyring_vault plugin (Failure)

How to run the script: