According to the Nikkei, Vietnam has informally approached Japan on the possibility of acquiring P-3Cs that the Japan Maritime Self-Defense Force (JMSDF) will be retiring.
By Los688 (my own work) [CC0], via Wikimedia Commons
Hanoi is looking north east for supplies of the P-3 as it feels that those manufactured by Kawasaki Heavy Industries are cheaper compared to getting them second-hand from Lockheed Martin. Japan is in the midst of swapping out the P-3 with the indigenous P-1, thus Vietnam thinks it will be easier to get its hands on a few of those maritime patrol planes.
Another advantage Vietnam hopes to tap on is Japan’s expertise in operating the P-3. The JMSDF has deployed its P-3s to Danang, Vietnam for exercises and the Vietnamese have experience working with Japanese crew.