Conversation so far:
What is the Mekong Delta?
In the 17th century a weakened Khmer state left the Mekong Delta poorly administered after repeated warfare with Siam.
What did Khmer do?
control of the area grew increasingly tenuous while increasing waves of Vietnamese settlers to the Delta isolated the Khmer of the Mekong Delta from the Cambodian kingdom.
When did Vietnam absorb it?
By 1757 the Vietnamese had absorbed the provinces of Psar Dek.
Were the any other provinces?
provinces of Psar Dek (renamed Sa Dec in Vietnamese) on the Mekong itself, and Moat Chrouk (Vietnamized to Chau Doc) on the Bassac River.
What else is significant about the Mekong Delta?
The legal transfer of control cut Cambodia off from the sea via the Mekong River as the Delta administered by an independent Vietnam.

Question: What was the impact of this?
Self-contained rewrite: What was the impact of The legal transfer of control cut Cambodia off from the sea via the Mekong River as the Delta administered by an independent Vietnam?