Just a word of caution from a S&R Tech (CASARA) on the reported 'oil-slicks' apparently seen by the Vietnamese. These waters are near some of the busiest shipping lanes in the world. Many vessels still routinely (and illegally) 'flush' their tanks while at sea. Yes, the sheen could be from aircraft Jet A1 - but would dissipate pretty quickly, in hours not days.
There is a distinct possibly it came from a passing vessel though...