During my rides in Laos I saw plenty of Vietnam registred motorbikes ridden buy young westerners type backpackers. And I think I saw a couple of the same in Cambodia. So I think it would be no problem to do so....
Strange question to be asking when borders remain closed and likely to stay closed (certainly for tourists) for the foreseeable future. After they reopen, no idea how entry requirements for people or vehicles will look like.
FYI, prior to covid, the entry regulations for Vietnamese bikes heading into Laos and Cambodia were as follows:
Laos - around half of the shared Lao-Viet borders would generally permit Vietnamese bikes in, though Lao bikes were generally not allowed to enter Vietnam. A few central Vietnamese crossings were a little difficult and Dien Bien Phu would be out too.
Cambodia - most crossings allow Viet bikes in but without paperwork or insurance.
Laos/Cambodia border - generally not allowed for Viet bikes crossing into Cambodia; may be possible heading into Laos.