This analysis of a dispute between foreign aircraft lessor, Fitzwalter Capital Partners, and Vietnam’s Vietjet, in which Vietjet failed to return four planes after a lease agreement was terminated, goes into the legal argument for keeping the aircraft in Vietnam.
Specifically, the article states that when the ownership of the lease was transferred and the aircraft were re-registered in Guernsey as opposed to Vietnam, it removed them from the Civil Aviation Authority of Vietnam’s jurisdiction–the CAAV. This meant that the CAAV could not issue an export licence and without one the aircraft cannot leave Vietnam legally.
On a side note, this article doesn’t mention Vietjet by name and spends a good deal of time suggesting there may have been nefarious motives behind the change of ownership of the original lease by its financiers. It does not, however, make any specific claims but rather asks very broad, vague questions.