So far, there are no known methods of downgrading a retail vita unit, (aside from replacing the motherboard).. Probably the only way is Manufacturing mode or some sort of QA flag. Alternatively you may try to do it by downgrading syscon fw, or if you could fully convert a retail into a testing kit.
However, it is possible to downgrade a Testing Kit or Development Kit Vita, as seen in this video:
TheFloW tried downgrade from 3.67 back to 3.60, but it didn't work https://twitter.com/theflow0/status/967785107888201728 https://imgur.com/dADtkam