It is true the historical Cambodian ambivalence over territorial claims on Koh Tral / Phu Quok as well as the use of such claims on political and nationalistic grounds that rest authority to those claims. However, Cambodia has the right to request a technical review of its claim over the rights on its encroached sea platform.
According to the United Nations Convention on the Law of the Sea “every State has the right to establish the breadth of its territorial sea up to a limit not exceeding 12 nautical miles” (UNCLS, art. 3)
We can determine that the base line in our case is the litoral of Kampot, Kep and Sihanouk provinces. It is not difficult to demonstrate that Koh Tral Island is right in front to the Cambodian base line. It is just 10 miles from the Cambodian coast, so it is nothing more than inside the Interior waters of Cambodia.
Vietnam argues something similar on its claim over the Spratlys islands on the South Chinese Sea, even establishing that they are nearer to Vietnam than China, Taiwan and the Philippines (see this report). At the same time, it defends that the islands have been historically Vietnamese, including its French colonial time. Koh Tral is almost attached to the Cambodian base line than the Spratlys. Reviewing the history of the Spratlys, they are actually islets in the middle of a sea that has been historically the center of several sea powers such as China, Malaysia, Vietnam, Japan, the colonial powers of France, US, etc. On historical bases, all have the right to claim them in different proportions.
The historical ambivalence of Cambodian population on Koh Tral does not give legal rights to Vietnam for the following reasons:
Vietnam has been an expansive power that historically comes from the north.
Cambodia is naturally the native of all this region, including the South of Vietnam (Kampuchea Kraom that translates “Low Cambodia”, like to say “Low California”.)
Cambodians, even during the ancestral Khmer Empire, never have been a sea power like Malaysia, Vietnam or China. It could be one of the reasons of its decline. This explains also an ancestral Cambodian indifference for the sea and why Cambodians did not show much interest in setting there. But this fact does not represent an objection to a legal claim.
Finally, one argument that should be reviewed as Koh Tral belonging to the Cambodian sea platform is Bokor Mountain. Everybody sees the Cambodian base line, but I never have seen this argument: Koh Tral is the sea continuation of the Bokor range. This natural phenomenon can be seen from any Kep beach.
Wait for a clear photo of this geographical evidence.