What large body of water separates Vietnam from the Philippine Island?

Depends on which neighbor you're talking about...

To the North between the Philippines and Taiwan is the Luzon Strait

To the East there are no more islands for a while

To the South the Celebes Sea separates the Philippines from Indonesia

To the SouthWest is the Sulu Sea that separates it from Malaysia

To the West is the South China that separates it from Vietnam