my money would be on HKG. before AA added the route in the past year, CX was the only airline that flew it.

all LAX transoceanic flying is done with the 789; this would put UA at a competitive disadvantage relative to CX/AA's premium cabins on a mature route that has plenty of HVFs. i don't see UA flying a 77W in a W-pattern just to get polaris seats on the route. UA's fleets (and even sub-fleets) are becoming increasingly 'locked' to one or two hubs.

if not HKG, then SGN would be my second choice. it fits the 'long, thin' profile of the 787, there's no nonstop competition on the route, and i believe LA has the largest vietnamese expat population in the US. 2015 PDEW for LAX-SGN was 279, compared to 174 for SFO-SGN (link to .pdf of OAG report). we all know UA is a lot more willing to start TPAC service ex-SFO compared to LAX, so new LAX additions probably require significantly better figures like that. vietnam's economy may have developed to the point where UA feels comfortable that they can fill the front of the plane, which is essential for longhaul routes.

BKK is my distant third place pick. the only reason why i even bother to mention BKK is that 1+ years ago, a LAX GS flyer with a solid reputation on this board said that he/she was told by a GS agent that a return to BKK for UA was imminent. OTOH, that was still in the smisek era, and a lot has changed since then.