The unofficial launch schedule for Call of Duty indicates that 2019's entry in the franchise is being made by Infinity Ward, the renowned makers of 2016's Call of Duty: Infinite Warfare, while next year's installment may be in the works at Sledgehammer Games, the studio behind 2017's Call of Duty: WWII.
While nothing has been confirmed yet, Gaming INTEL reports that 2020's Call of Duty is indeed being developed at Sledgehammer. According to their sources (the INTEL crew had leaked countless details on Black Ops IIII before Activision even unveiled the title), their game is "expected to be set in Vietnam or the Pacific Front," bringing the series back to Call of Duty: World at War and Black Ops' settings.
As revealed by INTEL, "this information comes from an incredibly early stage of the game’s development, and since informing Gaming INTEL our source has become disconnected from the project," but the simple fact that Sledgehammer was the studio that made the very first "historic" COD in ages gives hope that this Vietnam-based COD is indeed coming in 2020 – probably in November.
Sledgehammer has already tried to develop a Pluton-inspired (and quite interestingly, third-person) COD in the past, but Activision instead tasked them with co-development on 2011's Modern Warfare 3.
While that rumored title would definitely make longtime COD fans happy, remember that until anything is officially announced, consider everything reported on this subject nothing more than a gossip. At the time of writing, Activision had not revealed any plans to make a Vietnam-based COD.